The Danish National Birth Cohort--its background, structure and aim

Iron supplement use among Danish pregnant women

OBJECTIVES

To evaluate compliance with the national recommendation on supplemental iron to all pregnant women in Denmark and to explore differences between compliers and non-compliers with respect to dietary habits and other lifestyle factors.

DESIGN

Intake of supplemental iron from pure iron supplements and from multivitamin and mineral preparations was estimated in mid-pregnancy.

SETTING

Nationwide cohort study, the Danish National Birth Cohort (DNBC), comprising more than 100,000 women recruited in early pregnancy.

SUBJECTS

Information on diet and dietary supplements was available for 54,371 women. Of these, information on lifestyle factors was available for 50,902 women.

RESULTS

A high compliance with the recommendation was found, as approximately 77% of the women reported use of iron supplements during pregnancy. However, many of the compliers did not obtain the recommended doses of iron, which can partly be explained by the lack of iron preparations of appropriate doses available on the Danish market. Compliance with the recommendation was associated with age above 20 years, primiparity, body mass index<30 kg m- 2, non-smoking and long education. No major differences were seen in dietary intake between compliers and non-compliers.ConclusionOverall, a high compliance rate was found among participants of the DNBC but a clarification on daily dose is needed, and more concern should be paid to vulnerable groups such as young, smoking women and women with no or short education.

Exposición a contaminantes atmosféricos durante el embarazo y desarrollo prenatal y neonatal: protocolo de investigación en el proyecto INMA (Infancia y Medio Ambiente)

INTRODUCTION

The INMA (INfancia y Medio Ambiente [Spanish for Environment and Childhood]) project is a cooperative research network. This project aims to study the effects of environment and diet on fetal and early childhood development. This article aims to present the air pollutant exposure protocol during pregnancy and fetal and early childhood development of the INMA project.

METHODS

The information to assess air pollutant exposure during pregnancy is based on outdoor measurement of air pollutants (nitrogen dioxide [NO2], volatile organic compounds [VOC], ozone, particulate matter [PM10, PM2,5 ] and of their composition [polycyclic aromatic hydrocarbons]); measurement of indoor and personal exposure (VOC and NO2); urinary measurement of a biological marker of hydrocarbon exposure (1-hydroxypyrene); and data gathered by questionnaires and geographic information systems. These data allow individual air pollutant exposure indexes to be developed, which can then be used to analyze the possible effects of exposure on fetal development and child health.

CONCLUSION

This protocol and the type of study allow an approximation to individual air pollutant exposure to be obtained. Finally, the large number of participants (N = 4,000), as well as their geographic and social diversity, increases the study's potential.

Effect of reducing caffeine intake on birth weight and length of gestation: randomised controlled trial

OBJECTIVE

To estimate the effect of reducing caffeine intake during pregnancy on birth weight and length of gestation.

DESIGN

Randomised double blind controlled trial.

SETTING

Denmark.

PARTICIPANTS

1207 pregnant women drinking at least three cups of coffee a day, recruited before 20 weeks' gestation.

INTERVENTIONS

Caffeinated instant coffee (568 women) or decaffeinated instant coffee (629 women).

MAIN OUTCOME MEASURES

Birth weight and length of gestation.

RESULTS

Data on birth weight were obtained for 1150 liveborn singletons and on length of gestation for 1153 liveborn singletons. No significant differences were found for mean birth weight or mean length of gestation between women in the decaffeinated coffee group (whose mean caffeine intake was 182 mg lower than that of the other group) and women in the caffeinated coffee group. After adjustment for length of gestation, parity, prepregnancy body mass index, and smoking at entry to the study the mean birth weight of babies born to women in the decaffeinated group was 16 g (95% confidence interval -40 to 73) higher than those born to women in the caffeinated group. The adjusted difference (decaffeinated group-caffeinated group) of length of gestation was -1.31 days (-2.87 to 0.25).

CONCLUSION

A moderate reduction in caffeine intake in the second half of pregnancy has no effect on birth weight or length of gestation.

TRIAL REGISTRATION

Clinical Trials NCT00131690 [ClinicalTrials.gov].

Cryptorchidism and maternal alcohol consumption during pregnancy

BACKGROUND

Prenatal exposure to alcohol can adversely affect the fetus. We investigated the association between maternal alcohol consumption during pregnancy and cryptorchidism (undescended testis) among newborn boys.

METHODS

We examined 2,496 boys in a prospective Danish-Finnish birth cohort study for cryptorchidism at birth (cryptorchid/healthy: 128/2,368) and at 3 months of age (33/2,215). Quantitative information on alcohol consumption (average weekly consumption of wine, beer, and spirits and number of binge episodes), smoking, and caffeine intake was obtained by questionnaire and/or interview once during the third trimester of pregnancy, before the outcome of the pregnancy was known. For a subgroup (n = 465), information on alcohol consumption was obtained twice during pregnancy by interviews.

RESULTS

We investigated maternal alcohol consumption both as a continuous variable and categorized. The odds for cryptorchidism increased with increasing weekly alcohol consumption. After adjustment for confounders (country, smoking, caffeine intake, binge episodes, social class, maternal age, parity, maturity, and birth weight) the odds remained significant for women with a weekly consumption of five or more alcoholic drinks (odds ratio = 3.10; 95% confidence interval, 1.05-9.10).

CONCLUSIONS

Regular alcohol intake during pregnancy appears to increase the risk of congenital cryptorchidism in boys. The mechanisms for this association are unknown. Counseling of pregnant women with regard to alcohol consumption should also consider this new finding.

Maternal vaccination and preterm birth: using data mining as a screening tool

OBJECTIVE

The main purpose of this study was to identify possible associations between medicines used in pregnancy and preterm deliveries using data mining as a screening tool.

SETTINGS

Prospective cohort study.

METHODS

We used data mining to identify possible correlates between preterm delivery and medicines used by 92,235 pregnant Danish women who took part in the Danish National Birth Cohort (DNBC). We then evaluated the association between one of the identified exposures (vaccination) and the risk for preterm birth by using logistic regression. The women were classified into groups according to their exposure to vaccination. The regression analyses were adjusted for the following covariates: parity, infant's gender, maternal Body-Mass Index (BMI), age, smoking, drinking, job, number of inhabitants in the place of residence, infections, diabetes, high blood pressure and preeclampsia.

MAIN OUTCOME MEASURE

Preterm birth, a delivery occurring before the 259th day of gestation (i.e., less than 37 full weeks).

RESULTS

Data mining had indicated that maternal vaccination (among other factors) might be related to preterm birth. The following regression analysis showed that, the women who reported being vaccinated shortly before or during gestation had a slightly higher risk of giving preterm birth (O.R. = 1.14; 95% CI 1.04-1.25) as compared to the non-vaccinated group.

CONCLUSION

Whether the association between maternal vaccination and the risk for preterm birth found here is causal or not deserves further studies. Data mining, especially with additional refinements, may be a valuable and very efficient tool to screen large databases for relevant information which can be used in clinical and public health research.

Infertility, infertility treatment and twinning: the Danish National Birth Cohort

BACKGROUND

We have previously observed that an increasing time to pregnancy (TTP) is associated with a reduced frequency of twin deliveries in couples not receiving infertility treatment. By using updated information, we assessed the frequencies of dizygotic (DZ) and monozygotic (MZ) twin deliveries as a function of infertility (TTP > 12 months), as well as infertility treatment.

METHODS

From the Danish National Birth Cohort (1997-2003), we identified 51 730 fertile couples with TTP <or= 12 months, 5838 infertile couples who conceived naturally with TTP > 12 months and 5163 infertile couples who conceived after treatment. Information on zygosity, available for part of the cohort (1997-2000), was based on standardized questions on the similarities between the twins at the age of 3-5 years.

RESULTS

Compared with fertile couples, the frequency of DZ twin deliveries was lower for infertile couples conceiving naturally (odds ratio 0.4, 95% confidence interval 0.2-0.7) and was much higher for infertile couples conceiving after treatment (17.3, 14.4-20.7). The frequency of DZ twin deliveries decreased with TTP in untreated couples, whereas the frequency of MZ twin deliveries remained constant.

CONCLUSIONS

The frequency of DZ twin deliveries decreased with TTP and substantially increased with infertility treatment, whereas MZ twin deliveries remained substantially unchanged.

Low compliance with recommendations on folic acid use in relation to pregnancy: is there a need for fortification?

Objective:

As a means to prevent neural tube defects (NTDs), women planning pregnancy in Denmark are recommended to take a dietary supplement of 400 μg folic acid daily during the periconceptional period. We examined compliance with this recommendation in a national survey.

Design:

Cohort study on pregnant women in Denmark.

Setting:

The Danish National Birth Cohort (DNBC).

Subjects:

From November 2000 to February 2002, 22 000 pregnant women were recruited for DNBC. Use of dietary supplements was recorded at enrolment. Compliance with the recommendation was related to an information campaign that took place during the second half of 2001, and to lifestyle factors provided in a telephone interview by the end of the first trimester of pregnancy.

Results:

An increase was seen in the proportion of women complying with the recommendation in the study period and this coincided with the information campaign events. However, even at the end of the period, only 22.3% of the women who had planned their pregnancy fully complied with the recommendation. No increase at all was seen in periconceptional folic acid use among women with unplanned pregnancies. Young age, low education and smoking were identified as factors that determined non-compliance.

Conclusions:

Alternative and more effective strategies are needed if the Danish population is to benefit fully from the knowledge that folic acid prevents NTDs. Future strategies should not only target vulnerable groups, such as the less educated and the young, but also women who get pregnant without planning this. The only possible way to reach the last group may be through fortification of foods with folic acid.

Validity of protein, retinol, folic acid and n–3 fatty acid intakes estimated from the food-frequency questionnaire used in the Danish National Birth Cohort

Objective

To validate intakes of protein, folic acid, retinol and n–3 fatty acids estimated from a food-frequency questionnaire in week 25 of pregnancy (FFQ-25).

Design

Validation was done against a 7-day weighed food diary (FD) and biomarkers of the nutrients in gestation week 32–38.

Subjects and setting

The FFQ-25 to be validated was used in the Danish National Birth Cohort comprising 101 042 pregnant Danish women, of whom 88 participated in the present validation study.

Results

Estimated intakes of protein, retinol and folic acid did not differ significantly between the two dietary methods, but intake of n–3 fatty acids was one third larger when estimated from the FFQ-25. The intakes estimated from the two dietary methods were all significantly correlated, ranging from 0.20 for retinol intake to 0.57 for folic acid intake. Sensitivities of being correctly classified into low and high quintiles were between 0.22 and 0.77, and specificities were between 0.62 and 0.89. Urinary protein content did not correlate significantly with protein estimated from the FFQ (r = 0.17, P > 0.05), but did with intake estimated from the FD (r = 0.56, P < 0.0001). Erythrocyte folate correlated significantly with the estimated total intake from the FFQ (r = 0.55, P < 0.0001) and the FD (r = 0.52, P < 0.0001). No correlations with plasma retinol were found. Erythrocyte eicosapentaenoic acid (C20:5n-3) correlated significantly with n–3 fatty acids intake estimated from both the FFQ-25 (r = 0.37, P < 0.001) and the FD (r = 0.62, P < 0.0001).

Conclusion

The FFQ-25 gives reasonable valid estimates of protein, retinol and folic acid intakes, but seems to overestimate intake of n–3 fatty acids.

Does low participation in cohort studies induce bias?

BACKGROUND

Participation rates in large cohort studies have decreased during the last 2 decades. The consequences of this trend for relative risk estimation are unknown.

METHODS

The impact of a low participation rate (30%) on the Danish National Birth Cohort was examined among 49,751 women from the source population, including 15,373 participants in the cohort study. On the basis of independent data collection, we estimated odds ratios (ORs) in the source population and among participants for 3 exposure-risk associations: (a) in vitro fertilization and preterm birth, (b) smoking during pregnancy and birth of a small-for-gestational-age infant, and (c) prepregnancy body mass index and antepartum stillbirth. The effect of nonparticipation was described by a relative odds ratio (ROR), calculated as the OR(participants)/OR(source population). Two methods for calculation of confidence intervals for the relative odds ratio also were assessed.

RESULTS

The effect of nonparticipation on the selected ORs was small. The relative ORs were close to one and the bias was never larger than 16%, although some of the confidence intervals were wide. The 2 methods for calculation of confidence intervals gave very similar results and a small simulation study showed that the coverage probabilities were close to the 95% nominal level.

CONCLUSION

For the 3 chosen associations, the ORs were not biased by nonparticipation. The results are reassuring for studies based on the Danish cohort and similar cohorts of pregnant women. The methodology used to compute confidence intervals for the relative odds ratios performed well in the scenarios considered.

Prenatal exposure to loratadine in children with hypospadias: a nested case-control study within the Danish National Birth Cohort

The aim of this study was to examine the risk of hypospadias after reported exposure to loratadine and other antihistamines during pregnancy, based on data from the Danish National Birth Cohort. We examined the risk of hypospadias in a nested case-control design based on women enrolled in the Danish National Birth Cohort from 1998 to 2002 ( approximately 95,000 pregnant women). Data on maternal use of medicine in pregnancy were retrieved from questionnaires and telephone interviews, and data on birth outcomes were obtained from the Hospital Discharge Registry (HDR). Within the Danish National Birth Cohort, we identified cases with a diagnosis of hypospadias and randomly selected 10 controls per case without such a diagnosis (matched by date of birth). We identified 203 cases of hypospadias recorded in the HDR within 1 year postpartum and 2030 controls. One case (0.5%) and 25 (1.2%) controls reported exposure to loratadine in the first trimester or up to 30 days before the time of conception. The adjusted odds ratio (OR) for hypospadias among users of loratadine relative to nonusers was 0.9 (95% CI: 0.1-6.9) and the corresponding OR for other antihistamines was 0.5 (95% CI: 0.1-1.9). These data do not indicate an increased risk of hypospadias associated with maternal exposure to loratadine. In addition, this study does not suggest any risk differential between maternal exposure to loratadine and other antihistamines. However, the statistical precision of the risk estimates was low.

Atopic disposition, wheezing, and subsequent respiratory syncytial virus hospitalization in Danish children younger than 18 months: a nested case-control study

OBJECTIVES

This study examined whether atopic disposition, wheezing, and atopic disorders increased the risk of hospitalizations because of respiratory syncytial virus in children between birth and 18 months of age.

PATIENTS AND METHODS

Relative risks for respiratory syncytial virus hospitalization were studied in a nested 1:5 case-control design using exposure information obtained from interviews with mothers of 2564 case and 12,816 control children who had been followed prospectively from birth and until 18 months of age as participants in the Danish National Birth Cohort. Information on the children's ages at respiratory syncytial virus hospitalization, presentation of infrequent wheezing, recurrent wheezing, and atopic dermatitis were used to study these associations chronologically.

RESULTS

The adjusted relative risk of respiratory syncytial virus hospitalization in the offspring was 1.11 for maternal atopic dermatitis, 1.72 for maternal asthma, and 1.23 for paternal asthma. Atopic dermatitis in the child was associated with an increased risk of subsequent respiratory syncytial virus hospitalization among infants <6 months of age. Infrequent wheezing was associated with a relative risk of subsequent respiratory syncytial virus hospitalization of 2.98 and recurrent wheezing with a relative risk of 5.90. These associations were present also if infants with medical risk factors were excluded from the analysis.

CONCLUSIONS

Asthmatic disposition and wheezing were strong determinants of subsequent respiratory syncytial virus hospitalization in Danish children <18 months of age.

The National Children's Study: a 21-year prospective study of 100,000 American children

Prospective, multiyear epidemiologic studies have proven to be highly effective in discovering preventable risk factors for chronic disease. Investigations such as the Framingham Heart Study have produced blueprints for disease prevention and saved millions of lives and billions of dollars. To discover preventable environmental risk factors for disease in children, the US Congress directed the National Institute of Child Health and Human Development, through the Children's Health Act of 2000, to conduct the National Children's Study. The National Children's Study is hypothesis-driven and will seek information on environmental risks and individual susceptibility factors for asthma, birth defects, dyslexia, attention-deficit/hyperactivity disorder, autism, schizophrenia, and obesity, as well as for adverse birth outcomes. It will be conducted in a nationally representative, prospective cohort of 100,000 US-born children. Children will be followed from conception to 21 years of age. Environmental exposures (chemical, physical, biological, and psychosocial) will be assessed repeatedly during pregnancy and throughout childhood in children's homes, schools, and communities. Chemical assays will be performed by the Centers for Disease Control and Prevention, and banks of biological and environmental samples will be established for future analyses. Genetic material will be collected on each mother and child and banked to permit study of gene-environment interactions. Recruitment is scheduled to begin in 2007 at 7 Vanguard Sites and will extend to 105 sites across the United States. The National Children's Study will generate multiple satellite studies that explore methodologic issues, etiologic questions, and potential interventions. It will provide training for the next generation of researchers and practitioners in environmental pediatrics and will link to planned and ongoing prospective birth cohort studies in other nations. Data from the National Children's Study will guide development of a comprehensive blueprint for disease prevention in children.

Infertility, infertility treatment, and congenital malformations: Danish national birth cohort

OBJECTIVES

To examine whether infertile couples (with a time to pregnancy of > 12 months), who conceive naturally or after treatment, give birth to children with an increased prevalence of congenital malformations.

DESIGN

Longitudinal study.

SETTING

Danish national birth cohort.

PARTICIPANTS

Three groups of liveborn children and their mothers: 50,897 singletons and 1366 twins born of fertile couples (time to pregnancy < or = 12 months), 5764 singletons and 100 twins born of infertile couples who conceived naturally (time to pregnancy > 12 months), and 4588 singletons and 1690 twins born after infertility treatment.

MAIN OUTCOME MEASURES

Prevalence of congenital malformations determined from hospital discharge diagnoses.

RESULTS

Compared with singletons born of fertile couples, singletons born of infertile couples who conceived naturally or after treatment had a higher prevalence of congenital malformations-hazard ratios 1.20 (95% confidence interval 1.07 to 1.35) and 1.39 (1.23 to 1.57). The overall prevalence of congenital malformations increased with increasing time to pregnancy. When the analysis was restricted to singletons born of infertile couples, babies born after treatment had an increased prevalence of genital organ malformations (hazard ratio 2.32, 1.24 to 4.35) compared with babies conceived naturally. No significant differences existed in the overall prevalence of congenital malformations among twins.

CONCLUSIONS

Hormonal treatment for infertility may be related to the occurrence of malformations of genital organs, but our results suggest that the reported increased prevalence of congenital malformations seen in singletons born after assisted reproductive technology is partly due to the underlying infertility or its determinants. The association between untreated infertility and congenital malformations warrants further examination.

The Dietary Quality Score: validation and association with cardiovascular risk factors: the Inter99 study

OBJECTIVE

To develop and assess the validity of the Dietary Quality Score (DQS) and investigate the association with cardiovascular risk factors.

DESIGN

Cross-sectional population-based study.

SETTING

Copenhagen County, Glostrup, Denmark.

SUBJECTS

A random sample of 12,934 men and women aged 30-60 years were invited to a health examination. A total of 6542 participants were included in the statistical analysis.

METHODS

The DQS was developed using eight questions from a 48-item food frequency questionnaire (FFQ) and validated using a 198-item FFQ. Associations between the DQS and fasting serum lipids, homocysteine, blood pressure and the absolute risk of ischaemic heart disease (IHD) were explored.

RESULTS

A higher DQS was shown to be associated with higher dietary quality in general, including a low intake of fat, especially saturated fat; a high intake of fibre; various vitamins and minerals; and fruit, fish, vegetables and whole-grain products. A higher score according to the DQS was significantly negatively associated with total cholesterol (P=0.0031), triglyceride (P=0.0406), low-density lipoprotein-cholesterol (P=0.0071), homocysteine (P<0.0001) and the absolute risk of IHD (P<0.0001), adjusted for sex, age, smoking habits and physical activity level.

CONCLUSIONS

The DQS is a simple, valid and quick tool to make a rough classification of individuals into groups with high, average and low dietary quality. The DQS is negatively associated with serum lipids, homocysteine and the absolute risk of IHD.

SPONSORSHIP

The Inter99 study is supported economically by The Danish Medical Research Council, The Danish Centre for Evaluation and Health Technology Assessment, Novo Nordisk, Copenhagen County, The Danish Heart Foundation, The Danish Pharmaceutical Association, Augustinus Foundation, Ib Henriksen Foundation and Becket Foundation, Copenhagen County.

The Generation R Study: Design and cohort profile

The Generation R Study is a population-based prospective cohort study from fetal life until young adulthood. The study is designed to identify early environmental and genetic causes of normal and abnormal growth, development and health from fetal life until young adulthood. The study focuses on four primary areas of research: (1) growth and physical development; (2) behavioral and cognitive development; (3) diseases in childhood; and (4) health and healthcare for pregnant women and children. In total, 9778 mothers with a delivery date from April 2002 until January 2006 were enrolled in the study. Of all eligible children at birth, 61% participate in the study. Data collection in the prenatal phase included physical examinations, questionnaires, fetal ultrasound examinations and biological samples. In addition, more detailed assessments are conducted in a subgroup of 1232 pregnant women and their children. The children form a prenatally recruited birth-cohort that will be followed until young adulthood. Eventually, results forthcoming from the Generation R Study have to contribute to the development of strategies for optimizing health and healthcare for pregnant women and children.

The Generation R Study: Design and cohort profile

The Generation R Study is a population-based prospective cohort study from fetal life until young adulthood. The study is designed to identify early environmental and genetic causes of normal and abnormal growth, development and health from fetal life until young adulthood. The study focuses on four primary areas of research: (1) growth and physical development; (2) behavioral and cognitive development; (3) diseases in childhood; and (4) health and healthcare for pregnant women and children. In total, 9778 mothers with a delivery date from April 2002 until January 2006 were enrolled in the study. Of all eligible children at birth, 61% participate in the study. Data collection in the prenatal phase included physical examinations, questionnaires, fetal ultrasound examinations and biological samples. In addition, more detailed assessments are conducted in a subgroup of 1232 pregnant women and their children. The children form a prenatally recruited birth-cohort that will be followed until young adulthood. Eventually, results forthcoming from the Generation R Study have to contribute to the development of strategies for optimizing health and healthcare for pregnant women and children.

Fatty acid composition of human milk in atopic Danish mothers

BACKGROUND

Atopic dermatitis has been related to a disturbed metabolism of polyunsaturated fatty acids (PUFAs).

OBJECTIVE

We tested whether the PUFA composition of breast milk differs significantly between mothers with atopic dermatitis, mothers with other types of atopy, and nonatopic mothers. We also investigated whether differences in diet can explain possible observed differences.

DESIGN

Mothers with current or previous asthma (n = 396) were divided into 3 groups according to history of atopic dermatitis and allergic rhinitis. Breast-milk samples were collected from 314 women approximately 3 wk after delivery. The habitual diet of the women was assessed with food-frequency questionnaires in the 25th week of gestation (n = 207). Breast-milk samples and simultaneous dietary data from 14 nonatopic mothers were used for comparison.

RESULTS

Compared with the milk of nonatopic mothers, that of atopic mothers had significantly higher concentrations of 22:5n-6 and lower concentrations of 20:5n-3; moreover, 20:4n-6/20:5n-3, 22:5n-6/22:6n-3, and long-chain n-3 PUFA/18:3n-3 were shifted toward n-6 PUFA and 18:3n-3 in nonatopic and atopic mothers, respectively. No differences in breast-milk PUFA composition were evident between the subject groups. The diets of the groups differed only slightly with respect to protein intake. However, the PUFA composition of the breast milk was associated with diet and time of milk sampling, and the above difference in milk PUFAs disappeared when those factors were taken into account.

CONCLUSION

Our data do not support the possibility that the fatty acid composition of breast milk is affected by atopic dermatitis or atopy in general, because most differences in breast-milk PUFA composition appear to be explained by the diet.

Stillbirth and slow metabolizers of caffeine: comparison by genotypes

BACKGROUND

Cytochrome P4501A2 (CYP1A2) and N-acetyltransferase 2 (NAT2) are key enzymes in the metabolism of caffeine. The polymorphism of these genes facilitates the detection of fast and slow metabolizers, and if caffeine is causally related to stillbirth, we expect slow metabolizers to have a higher risk of stillbirth at any given intake of caffeine. Gluthatione S-transferase alpha1 (GSTA1) may also be active in the metabolism of caffeine as it conjugates glutathione to aromatic amines. Our study, therefore, included analyses of the association between GSTA1 and stillbirth.

METHODS

A nested case non-case study among women who participated in the Danish National Birth Cohort: 142 cases of singleton stillbirths and 157 controls of singleton live births.

RESULTS

Slow oxidizer status (CYP1A2), slow acetylator status (NAT2), and low activity of GSTA1 were not individually associated with the risk of stillbirth [odds ratio (OR) = 1.06, 95% confidence interval (95% CI) 0.67-1.67, OR = 0.95, 95% CI 0.60-1.51, and OR = 1.42, 95% CI 0.88-2.28, respectively]. We did, however, observe that subjects with a combination of slow CYP1A2, slow NAT2, and low GSTA1 genes had almost a 2-fold risk of stillbirth compared with subjects with other combinations of genotypes.

CONCLUSIONS

We found no link between any single genotype and the risk of stillbirth. An association between a combination of genotypes and stillbirth was discovered. Caffeine may be causally related to stillbirth, but larger studies using Mendelian randomization are needed to verify this.

The association between atopy and factors influencing folate metabolism: is low folate status causally related to the development of atopy?

BACKGROUND

Deficiency of folate has been associated with several disorders characterized by enhanced activation of the cellular immune system (non-allergic th1 type immune response). Whether folate status is also associated with atopic disease (allergic th2 type immune response) is unknown. We aimed at examining the association between atopy and markers of impaired folate metabolism, i.e. MTHFR(C677T) genotype, plasma total homocysteine, and dietary intakes of methionine, folates, and vitamins B12, B6, and B2.

METHODS

Cross-sectional population-based study of 1,671 male and female residents of Copenhagen County, Denmark, aged 30-60 years participating in a health examination during 1999-2001. Atopy was defined as positive levels of specific IgE against a panel of inhalant allergens. MTHFR(C677T) genotype was determined by PCR followed by restriction fragment length polymorphism analyses. Total homocysteine was measured by fluorescent polarization immunoassay. Dietary vitamin intakes were estimated from a semi-quantitative food frequency questionnaire.

RESULTS

The prevalence of atopy was associated with MTHFR(C677T) genotype. TT individuals had a significantly higher risk of atopy compared with CC/CT individuals [odds ratio 1.76, 95% confidence interval (95% CI) 1.19-2.60]. Additionally, gene-diet interaction effects were identified. Dietary markers were negatively associated with risk of atopy in persons with the TT genotype. Total homocysteine was not related to atopy (odds ratio per 5 mumol/l = 1.12, 95% CI 0.98-1.29).

CONCLUSIONS

The results suggest that an impaired folate metabolism may be causally related to the development of atopy.

Maternal fish oil supplementation during lactation does not affect blood pressure, pulse wave velocity, or heart rate variability in 2.5-y-old children

Maternal (n-3) PUFA deficiency is associated with higher blood pressure (BP) later in life in rat offspring, and early intake of (n-3) PUFA in formula-fed infants was shown to modify later BP. BP, heart rate (HR), and heart rate variability (HRV) are affected by dietary (n-3) PUFA in adults. In this study, we investigated whether fish oil (FO) supplementation of lactating mothers could modify BP, pulse wave velocity (PWV), and HRV in their children after 2 y. Mothers with low fish intake were randomly assigned to FO or olive oil (OO) supplementation for the first 4 mo after delivery. A reference group of mothers with a high habitual fish intake (HFI) was also followed. At the follow-up study at 2.5 y of age, BP and PWV were measured, and electrocardiograms were recorded for 0.5 h. FO supplementation significantly increased RBC levels of long-chain (n-3) PUFA of the 4 mo-old children, but at 2.5 y, the FO and OO groups did not differ. BP, PWV, HR, and HRV also did not differ among the groups. However, for all 3 groups, the children's intake of (n-3) PUFA at 2.5 y was negatively correlated with mean arterial pressure after adjustment for outdoor temperature (r = -0.245, P = 0.04). In conclusion, maternal FO supplementation had no overall effect on BP, PWV, or HRV of the children, indicating that (n-3) PUFA intake of Danish mothers may be sufficient in this sense. However, children's dietary intake of (n-3) PUFA might have a beneficial effect on BP in childhood.

Do women give the same information on binge drinking during pregnancy when asked repeatedly?

OBJECTIVE

To study if pregnant women give the same answers to questions on frequency and timing of binge drinking when asked more than once during and after pregnancy.

DESIGN

Cohort study.

SETTING

The Danish National Birth Cohort.

SUBJECTS

The study is based on 76 307 pregnant women with repeated information on binge drinking during the early part of pregnancy and 8933 pregnant women with information on binge drinking during pregnancy weeks 30-36, obtained while pregnant and 6 months after delivery.

RESULTS

More women reported binge drinking, if the interview took place close to the period in question. As the report of binge drinking was highest in the first of two interviews referring to the same period, as well as women who participated in the first interview in pregnancy week 12 or earlier reported more binge drinking compared to women who participated in the interview later in pregnancy.

CONCLUSIONS

Self-reported information on binge drinking is more frequently under-reported when the recall period is long. To improve the validity of data on binge drinking, future birth cohorts should obtain information several times during pregnancy.

Occupational Exposure to Pesticides and Pregnancy Outcomes in Gardeners and Farmers: A Study Within the Danish National Birth Cohort

OBJECTIVE

We conducted a follow-up study to examine whether exposure to pesticides during pregnancy had an adverse effect on pregnancy outcomes among Danish gardeners and farmers.

METHODS

Using data from the National Birth Cohort in Denmark, we identified 226 pregnancies of gardeners and 214 pregnancies of farmers during 1997 through 2003. Work activities and exposure to pesticides were reported in an interview (around 16 weeks of gestation). Totally, 62,164 other workers in the cohort served as a reference group. Pregnancy outcomes were obtained by linkage to the national registers. Cox regression was applied to analyze late fetal loss and congenital malformations, and logistic regression was used to analyze preterm birth and small for gestational age.

RESULTS

There were no significant differences in the studied pregnancy outcomes between gardeners or farmers and all other workers, except for an increased risk of very preterm birth for gardeners and a favorable birth weight for farmers. With the exception of biologic approach used in gardening, neither work activities nor exposure to pesticides showed a significant increased risk of adverse birth outcomes among gardeners or farmers.

CONCLUSIONS

Our data suggest little effect of occupational exposures to pesticides on pregnancy outcomes among gardeners or farmers in Denmark. The results may not apply to other countries.

General obstetrics: Fish oil in various doses or flax oil in pregnancy and timing of spontaneous delivery: a randomised controlled trial

OBJECTIVES

To test the earlier suggested hypothesis that intake of long-chain n-3 fatty acids from fish oil may delay the timing of spontaneous delivery and to test if alpha-linolenic acid, provided as flax oil capsules, shows the same effect.

DESIGN

Randomised controlled trial including women reporting low dietary fish intake. The women were allocated in the proportions of 1:1:1:1:1:1:2 into six treatment groups and a control group, respectively, from week 17-27 of gestation. The treatment groups received fish oil, in various doses, or flax oil, and the control group did not receive any treatment.

SETTING

The Danish National Birth Cohort.

SAMPLE

A total of 3098 women allocated into six treatment groups and one control group.

METHODS

The six intervention groups were offered fish oil capsules in doses of 0.1, 0.3, 0.7, 1.4 and 2.8 g of eicosapentaenoic acid and docosahexaenoic acid per day or 2.2 g of alpha-linolenic acid (ALA) per day from week 17-27 of gestation until delivery.

MAIN OUTCOME MEASURES

Timing of spontaneous delivery.

RESULTS

No differences in timing of spontaneous delivery was detected in the fish oil groups or the flax oil group, compared with the control group. The difference in timing of spontaneous delivery in the group receiving the highest fish oil dose compared with the control group was 0.8 days (95% CI: -2.3 to 1.0). Only a minority of the women in the intervention groups took capsules until delivery.

CONCLUSION

Possible explanations for these findings include no true effect of n-3 fatty acids on spontaneous delivery or a quick-acting effect not detectable in this trial.

Smoking habits, nicotine use, and congenital malformations

OBJECTIVE

We examined whether maternal smoking and use of nicotine substitutes during the first 12 weeks of pregnancy increased the prevalence of congenital malformations in general and of certain congenital malformations in particular.

METHODS

In the Danish National Birth Cohort (1997-2003) we identified 76,768 pregnancies (and their subsequent singleton births); 20,603 were exposed to tobacco smoking during the first 12 weeks of pregnancy. Birth outcomes were collected by linkage to the Central Population Register, the National Patients Register, and the National Birth Register. We identified congenital malformations from the Hospital Medical Birth Registry as they were recorded at birth or in the first year of follow-up.

RESULTS

Smoking mothers were younger, weighed less, consumed more alcohol, and had received less education. Children exposed to prenatal tobacco smoking had no increase in congenital malformations prevalence compared with the nonexposed children in both crude and adjusted analyses. Children born to nonsmokers, but who used nicotine substitutes, had a slightly increased relative congenital malformations prevalence ratio; relative prevalence rate ratio was 1.61 (95% confidence interval 1.01-2.58), which represents a 60% increased risk. When the analysis was restricted to musculoskeletal malformations, the relative prevalence rate ratio was 2.63 (95% confidence interval 1.53-4.52).

CONCLUSION

Our results showed no increase in congenital malformations related to prenatal tobacco smoking. However, we identified an increase of malformations risk in nonsmokers using nicotine substitutes. This finding needs to be replicated in other data sources.

LEVEL OF EVIDENCE

II-2.

Laboratory work and pregnancy outcomes: a study within the National Birth Cohort in Denmark

AIMS

To examine pregnancy outcomes in women doing laboratory work.

METHODS

Using data from the Danish National Birth Cohort (1997-2003), the authors conducted a prospective cohort study of 1025 female laboratory technicians and 8037 female teachers (as reference). The laboratory technicians were asked about laboratory work tasks during pregnancy in an interview (at around 16 weeks of gestation). Pregnancy outcomes were obtained by linking the cohort to the national registers. Hazard ratios (HRs) of late fetal loss and diagnosing of congenital malformations were calculated by using Cox regression, and odds ratios (ORs) of preterm birth and small for gestational age were calculated by using logistic regression.

RESULTS

Overall, there were no significant differences in pregnancy outcomes between laboratory technicians and teachers. However, we found that laboratory technicians working with radioimmunoassay or radiolabelling had an increased risk of preterm birth (OR = 2.2, 95% CI 0.8 to 6.2 for radioimmunoassay, and OR = 1.9, 95% CI 0.8 to 4.6 for radiolabelling) and "major" malformations (HR = 2.1, 95% CI 1.0 to 4.7 for radioimmunoassay, and HR = 1.8, 95% CI 0.9 to 3.7 for radiolabelling). The ORs of preterm birth doubled for women working with these tasks every day or several times a week. When an exposure matrix was applied, an increased risk of "major" malformations for exposure to organic solvents was seen.

CONCLUSIONS

The results did not indicate any high risk of reproductive failures in laboratory technicians in general. Exposure to radioisotopes may carry a high risk of preterm birth and congenital malformations. This finding deserves further investigation.

Fish oil supplementation of lactating mothers affects cytokine production in 2 1/2-year-old children

n-3 PUFA influence immune functioning and may affect the cytokine phenotype during development. To examine whether maternal fish oil supplementation during lactation could modify later immune responses in children, 122 lactating Danish mothers with a fish intake below the population median were randomized to groups supplemented for the first 4 mon of lactation with 4.5 g/d of fish oil (equivalent to 1.5 g/d of n-3 long-chain PUFA) or olive oil. Fifty-three mothers with a fish intake in the highest quartile of the population were also included. The FA composition of erythrocyte membranes was measured at 4 mon and at 2 1/2 yr. Plasma immunoglobulin E (IgE) levels and cytokine production in lipopolysaccharide-stimulated whole-blood cultures were determined at 2 1/2 yr. Erythrocyte n-3 PUFA at 4 mon were higher in infants from the fish oil group compared with the olive oil group (P < 0.001) but were no longer different at 2 1/2 yr. The median production of lipopolysaccharide-induced interferon gamma (IFN-gamma) in the fish oil group was fourfold higher than that in the olive oil group (P = 0.034), whereas interleukin-10 (IL-10) production was similar. The IFN-gamma/IL-10 ratio was twofold higher in the fish oil group (P = 0.019) and was positively correlated with 20:5n-3/20:4n-6 in erythrocytes at 4 mon (P = 0.050). The percentages of atopic children and plasma IgE were not different in the two groups, but the study was not designed to look at atopy. Cytokine responses and erythrocyte FA composition in children of mothers with a high fish intake were intermediate in comparison with those in the randomized groups. Fish oil supplementation during lactation resulted in increased in vitro IFN-gamma production in the children 2 yr after the supplementation was given, which may reflect a faster maturation of the immune system.

Maternal underweight and the risk of spontaneous abortion

BACKGROUND

To evaluate the risk of spontaneous abortion in relation to maternal pre-pregnant underweight.

METHODS

The study was designed as a cohort study within the framework of the Danish National Birth Cohort (DNBC). The participants were a total of 23 821 women recruited consecutively to the DNBC from October 1, 1997 to March 31, 1999 and interviewed subsequently. The pregnant women were recruited in first half of pregnancy and interviewed about pre-pregnant body size, obstetric history, exposures in pregnancy, and socio-demographic factors. Pregnancies were followed-up regarding spontaneous abortion. Relative risk of spontaneous abortion was calculated as Hazard Ratios using Cox regression with delayed entry.

RESULTS

The outcome measure was spontaneous abortion. The hazard ratio for spontaneous abortion in women with a pre-pregnant body mass index (BMI) below 18.5 was 1.24 (95% confidence limits 0.95-1.63) compared to women with pre-pregnant BMI 18.5-24.9. Women with a BMI of 25 or more had a smaller increase in risk of spontaneous abortion. Adjustment for maternal age, parity, previous miscarriages, and lifestyle factors did not affect the estimates substantially, neither did exclusion of women with metabolic or eating disorders.

CONCLUSION

These results indicate that pre-pregnant underweight may affect the risk of spontaneous abortion negatively.

Coffee and fetal death: a cohort study with prospective data

The authors conducted a cohort study within the Danish National Birth Cohort to determine whether coffee consumption during pregnancy is associated with late fetal death (spontaneous abortion and stillbirth). A total of 88,482 pregnant women recruited from March 1996 to November 2002 participated in a comprehensive interview on coffee consumption and potentially confounding factors in pregnancy. Information on pregnancy outcome was obtained from the National Hospital Discharge Register and medical records. The authors detected 1,102 fetal deaths. High levels of coffee consumption were associated with an increased risk of fetal death. Relative to nonconsumers of coffee, the adjusted hazard ratios for fetal death associated with coffee consumption of 1/2-3, 4-7, and > or =8 cups of coffee per day were 1.03 (95% confidence interval (CI): 0.89, 1.19), 1.33 (95% CI: 1.08, 1.63), and 1.59 (95% CI: 1.19, 2.13), respectively. Reverse causation due to unrecognized fetal demise may explain the association between coffee intake and risk of fetal death prior to 20 completed weeks' gestation but not the association with fetal loss following 20 completed weeks' gestation. Consumption of coffee during pregnancy was associated with a higher risk of fetal death, especially losses occurring after 20 completed weeks of gestation.

Psychosocial and physical work environment, and risk of pelvic pain in pregnancy. A study within the Danish national birth cohort

OBJECTIVE

The sparse knowledge of the aetiology of pelvic pain in pregnancy makes evidence based prevention a limited option. The aim of this study was to examine the relation between pelvic pain in pregnancy and physical and psychosocial working conditions.

METHODS

This study used self reported data on working conditions for 1219 cases and 1539 controls, sampled as a nested case-control study within the Danish national birth cohort. Cases and controls were selected on the basis of self reported pelvic pain intensity, pain localisation, and pain impact on daily living activities. Exposure data were collected prospectively; early in pregnancy and before the onset of pelvic pain. Main outcome measures were odds ratios for pelvic pain in pregnancy as a function of physical and psychosocial working conditions.

RESULTS

Pregnant women with fixed evening work and with rotating shifts (without night shift) had odds ratios for pelvic pain in pregnancy of 1.76 (95% confidence intervals 1.04 to 2.96) and 1.65 (1.22 to 2.24), respectively, compared with women with day work. Physically strenuous work was associated with an almost 50% increased risk of pelvic pain in pregnancy (1.47; 1.17 to 1.84). In women who were under high psychosocial strain at work odds ratio was 1.39 (1.12 to 1.74) compared with women with low job strain.

CONCLUSION

Both physically and psychosocially demanding working conditions, measured by physically strenuous work, rotating shifts, and high job strain, are associated with an increased reporting of pelvic pain in pregnancy.

Two definitions of "small size at birth" as predictors of motor development at six months

BACKGROUND

Small babies are known to be at increased risk for a wide range of difficulties. In predicting risk, it may be more informative to estimate smallness in relation to family norm (using the birth weight of an older sibling) rather than to use the standard "small-for-gestational-age" (SGA z-score) measure.

METHODS

For 10,577 babies born to women enrolled in the Danish National Birth Cohort, we calculated a "birth-weight ratio" (actual birth weight/expected birth weight predicted from older sibling x 100). We identified babies in the lowest decile of the birth-weight ratio (</=87.7% for boys and </=87.6% for girls) and compared them with babies in the lowest decile of the sex- and gestational age-specific distribution (SGA z-score). We evaluated how these definitions predicted motor development. We also compared how selected predictors of birth weight influenced the classification of the baby according to the birth-weight ratio and according to the SGA z-score.

RESULTS

Birth-weight ratio and SGA identified 1058 and 1059 babies, respectively, with 738 identified by both methods. A low birth-weight ratio predicted delayed motor development slightly better than did SGA. Babies classified as too small solely by the SGA criterion were more often born to small or smoking mothers than those identified solely by the birth-weight ratio.

CONCLUSIONS

The combined use of the birth-weight ratio and SGA may provide a more sensitive tool for identifying babies at risk. The birth-weight ratio is less likely to classify babies as small based on their mothers' body size, identifying instead babies who were substantially smaller than their sibling.

Prepregnancy obesity and fetal death: a study within the Danish National Birth Cohort

OBJECTIVE

To examine the association between high prepregnancy body mass index and fetal death, allowing for the effects of gestational age, weight gain, and maternal diseases in pregnancy.

METHODS

Prepregnancy body mass index (BMI) and fetal death were examined in the Danish National Birth Cohort among 54,505 pregnant women who participated in a comprehensive interview during the second trimester. Pregnancy outcomes were obtained from registers and medical records. Cox regression analyses with delayed entry and time-dependent covariates were used to estimate the risk of fetal death.

RESULTS

Compared with normal-weight women (18.5 < or = BMI < 25), the risks of fetal death among obese women (BMI > or = 30), expressed as adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) were as follows: before week 14: 0.8 (0.5-1.4), weeks 14-19: 1.6 (1.0-2.5), weeks 20-27: 1.9 (1.1-3.3), weeks 28-36: 2.1 (1.0-4.4), weeks 37-39: 3.5 (1.9-6.4), and weeks 40+: 4.6 (1.6-13.4). Overweight women (25 < or = BMI < 30) also experienced a higher risk after 28 weeks, and especially after 40 weeks of gestation (HR 2.9, 95% CI 1.1-7.7). Analysis of stillbirth (fetal death at 28+ completed weeks of gestation) indicated that the effects were not due to obesity-related diseases in pregnancy, nor was weight gain associated with stillbirth. The increased risk of stillbirth among overweight and obese women was partly attributable to inadequate placental function (crude odds ratios 2.1, 95% CI 1.0-4.4, and 5.2, 95% CI 2.5-10.9, respectively).

CONCLUSION

Prepregnancy obesity was associated with an increasing excess risk of fetal death with advancing gestation, and placental dysfunction may be a possible contributing factor.

Maternal fish oil supplementation in lactation and growth during the first 2.5 years of life

Fish oil addition to infant formulas has raised concern on whether increased intake of n-3 long-chain polyunsaturated fatty acid (n-3LCPUFA) affects infant growth. The objective of this study was to determine whether maternal fish oil supplementation during 0-4 mo of lactation influences growth in infancy and early childhood. In a randomized, blinded intervention trial, lactating Danish mothers with a fish intake below the population median were randomized to 4.5 g/d fish oil or olive oil. A reference group of 53 mothers with a fish intake in the highest quartile of the population and their infants were included in the study. Head circumference, weight, length, skinfold thickness, and waist circumference of children were measured at 2, 4, and 9 mo and at 2.5 y. One hundred children completed the intervention trial, and 72 were followed up at 2.5 y together with 29 from the reference group. Growth in weight, length, and head circumference did not differ between the randomized groups up to 9 mo, but at 2.5 y, body composition differed significantly. Children in the fish oil group had larger waist circumference body mass index (BMI; 0.6 kg/m(2); p = 0.022), and head circumference compared with those in the olive oil group. Adjusted for sex, ponderal index at birth and current energy intake, BMI at 2.5 y was associated with docosahexaenoic acid in maternal erythrocytes after the intervention. In conclusion, the n-3LCPUFA intake of lactating mothers may be important for growth of young children. The long-term effect on weight and BMI remains to be investigated.

Sibship characteristics and risk of allergic rhinitis and asthma

Studying associations between sibship characteristics and allergic diseases in detail may contribute clues to their etiology. The authors studied associations between sibship characteristics and risk of self-reported allergic rhinitis and asthma among 31,145 pregnant women participating in a nationwide study in Denmark during 1997-2000. Increasing sibship size was associated with a decreased risk of allergic rhinitis and asthma with allergic rhinitis but not with asthma without allergic rhinitis. The protective effect of having older siblings was stronger than the protective effect of having younger siblings for both allergic rhinitis and asthma with allergic rhinitis. There was no association between interval to closest older or younger sibling and risk of allergic rhinitis or asthma with allergic rhinitis, while the risk of asthma without allergic rhinitis increased with intervals of 2 or more years compared with less than 2 years to the nearest older sibling. The protective effect of having siblings on the risk of asthma with allergic rhinitis could be explained by a protective effect of siblings on the risk of allergic rhinitis alone. In conclusion, our findings suggest that different etiologic mechanisms are involved for allergic rhinitis and asthma with respect to the effect of sibship characteristics. Furthermore, the findings that allergic rhinitis was associated with the number of younger siblings but not with the age interval to younger siblings support the hypothesis of an influence of postnatal mechanisms and suggest that these mechanisms may not necessarily be operating only in early life.

Shift work, job stress, and late fetal loss: The National Birth Cohort in Denmark

OBJECTIVE

The Danish National Birth Cohort (DNBC) was used to examine whether shift work or job stress correlate with late fetal loss.

METHODS

We identified 33,694 pregnancies of daytime workers and 8,075 pregnancies of shift workers in women recruited to the DNBC between 1998 and 2001. Pregnancy outcomes were obtained by linkages to the national registers. Hazard ratios of fetal loss were calculated by using Cox regressions with left truncation.

RESULTS

Fixed night work was associated with fetal loss (hazard ratio = 1.85, 95% confidence interval = 1.00-3.42). No high risk of fetal loss was seen for other types of shift work. Job stress, as measured in our study, was not associated with fetal loss.

CONCLUSIONS

Our results suggest that fixed night work during pregnancy increases the risk of late fetal loss.

Maternal fish oil supplementation in lactation: effect on visual acuity and n-3 fatty acid content of infant erythrocytes

Studies on formula-fed infants indicate a beneficial effect of dietary DHA on visual acuity. Cross-sectional studies have shown an association between breast-milk DHA levels and visual acuity in breast-fed infants. The objective in this study was to evaluate the biochemical and functional effects of fish oil (FO) supplements in lactating mothers. In this double-blinded randomized trial, Danish mothers with habitual fish intake below the 50th percentile of the Danish National Birth Cohort were randomized to microencapsulated FO [1.3 g/d long-chain n-3 FA (n-3 LCPUFA)] or olive oil (OO). The intervention started within a week after delivery and lasted 4 mon. Mothers with habitual high fish intake and their infants were included as a reference group. Ninety-seven infants completed the trial (44 OO-group, 53 FO-group) and 47 reference infants were followed up. The primary outcome measures were: DHA content of milk samples (0, 2, and 4 mon postnatal) and of infant red blood cell (RBC) membranes (4 mon postnatal), and infant visual acuity (measured by swept visual evoked potential at 2 and 4 mon of age). FO supplementation gave rise to a threefold increase in the DHA content of the 4-mon milk samples (P < 0.001). DHA in infant RBC reflected milk contents (r = 0.564, P < 0.001) and was increased by almost 50% (P < 0.001). Infant visual acuity was not significantly different in the randomized groups but was positively associated at 4 mon with infant RBC-DHA (P = 0.004, multiple regression). We concluded that maternal FO supplementation during lactation did not enhance visual acuity of the infants who completed the intervention. However, the results showed that infants with higher RBC levels of n-3 LCPUFA had a better visual acuity at 4 mon of age, suggesting that n-3 LCPUFA may influence visual maturation.

Maternal prepregnant body mass index, duration of breastfeeding, and timing of complementary food introduction are associated with infant weight gain

BACKGROUND

Women who are overweight or obese before pregnancy breastfeed for shorter durations than do normal-weight women. These shorter durations may place infants of overweight and obese women at risk of not receiving the benefits of breastfeeding, which may include a reduced risk of overweight later in life.

OBJECTIVE

We examined how maternal prepregnant body mass index (BMI; in kg/m2) and infant feeding pattern are associated with infant weight gain.

DESIGN

In this prospective, observational study, we used multiple regression analyses adjusted for potential confounding factors to examine these associations among 3768 mother-infant dyads from the Danish National Birth Cohort.

RESULTS

In multiple regression analyses, increasing maternal prepregnant BMI, decreasing durations of breastfeeding, and earlier complementary food introduction were associated with increased infant weight gain. An interaction was identified for short durations of breastfeeding (<20 wk). Earlier complementary food introduction (<16 wk) was associated with greater infant weight gain; however, the timing of complementary food introduction did not increase infant weight gain at longer durations of breastfeeding (> or =20 wk). In this sample, prepregnant obesity (BMI > or = 30.0), short durations of breastfeeding, and earlier introduction of complementary food were associated with 0.7 kg of additional weight gain during infancy.

CONCLUSIONS

Infant weight gain is associated with maternal prepregnant BMI and with an interaction between the duration of breastfeeding and the timing of complementary food introduction. Future investigations of the effects of breastfeeding on infant weight gain should account for all of these factors.

The Copenhagen Prospective Study on Asthma in Childhood (COPSAC): design, rationale, and baseline data from a longitudinal birth cohort study

BACKGROUND

The atopic diseases asthma, atopic dermatitis, and allergic rhinitis are the most common chronic diseases in children, and their prevalence has increased recently in industrialized nations. Little is known about the genetic-environmental interaction factors driving such proliferation.

OBJECTIVE

To investigate the relationships among genetic, environmental, and lifestyle factors in the development of atopic diseases in high-risk children with the aim of developing evidence-based prevention strategies.

METHODS

The Copenhagen Prospective Study on Asthma in Childhood is a single-center, birth cohort study of children of asthmatic mothers. Objective assessments begin at birth, with scheduled visits every 6 months and when acute symptoms manifest. Clinical outcomes comprise preasthma, asthma, atopic dermatitis, allergic rhinitis, allergy, lung function, and bronchial responsiveness. Exposure assessments comprise respiratory, intestinal, and skin microbiology; the child's diet; indoor and outdoor air quality; allergens; and indicators of lifestyle. Genetic characteristics of probands and parents are evaluated. Quality assurance follows Good Clinical Practice guidelines.

RESULTS

Four hundred eleven infants of asthmatic mothers were enrolled at the age of 1 month. The children were born between August 2, 1998, and December 28, 2001. Compared with the Copenhagen population, mothers of the cohort population were less likely to have given natural childbirth. The households were slightly less affluent, with fewer children and fewer pets. Whites may be overrepresented. At age 2 years, 93% of the infants were still actively participating in the cohort.

CONCLUSIONS

This longitudinal birth cohort study of high-risk Danish infants consists of objective phenotyping, detailed information on exposure, high data quality, and a high participant retention rate.

Correlation between self-reported gestational age and ultrasound measurements

BACKGROUND

We studied the agreement between different measurements of gestational age, i.e. self-reported gestational age in the Danish National Birth Cohort Study, ultrasound-estimated gestational age from the medical records in one Danish county and gestational age from the Danish National Hospital Discharge Register.

METHODS

The ultrasound-estimated gestational length was based on the size of the biparietal diameter. The ultrasound-estimated gestational length was related to corrected and uncorrected last menstrual period estimates in the Danish National Cohort Study, and to the gestational length recorded in the Danish National Discharge Register. Non-parametric statistics were used in the analysis.

RESULTS

The gestational ages estimated by ultrasound were 2-3 days shorter than gestational ages estimated by the other methods. The gestational ages recorded by the Discharge Register and the gestational ages based on corrected last menstrual period did not differ significantly.

CONCLUSION

The self-reported gestational age in The Danish National Birth Cohort is in good concordance both with data from the National Hospital Discharge Register and with ultrasound-estimated gestational age.

Fecundity and twinning. A study within the Danish National Birth Cohort

BACKGROUND

Decreased fecundity has been hypothesized as one of the causes of the widespread decline in twin rates in the 1960s and 1970s. The association between high fecundity and twinning is not, however, well documented.

METHODS

In the Danish National Birth Cohort, we identified 56 295 births (including 729 twin deliveries) with data on pregnancy planning and time to pregnancy (TTP) and no report of infertility treatment. We estimated the odds ratio (OR) of twinning as a function of TTP and having planned a pregnancy.

RESULTS

Increasing TTP correlated with decreased twinning rates. Compared with women who conceived within 2 months of trying, women who took longer than 1 year had an OR of 0.55 (95% confidence interval 0.39-0.77) of giving birth to twins. The association was slightly stronger for opposite sex twins. Women not planning a pregnancy had a lower rate of opposite sex twins than planners, but a comparable rate of same sex twins. We saw an excess of same sex pairs in women conceiving despite contraception.

CONCLUSIONS

A long TTP is associated with a reduced risk of twinning. The marked decline observed in the 1960s and 1970s may derive from a widespread decrease in fecundity.

Animal protein intake, serum insulin-like growth factor I, and growth in healthy 2.5-y-old Danish children

BACKGROUND

Studies from developing countries indicate that intake of animal protein, especially of milk, is associated with greater velocity of linear growth in childhood. Whether the same association exists in industrialized countries, where protein intake is high, is not clear.

OBJECTIVE

Our objective was to examine associations between protein intake, serum insulin-like growth factor I (sIGF-I) concentrations, and height in healthy children.

DESIGN

We analyzed the associations between protein intake, sIGF-I concentrations, and height in 2.5-y-old children. Diet (7-d record) and sIGF-I (radioimmunoassay) data were available from 90 children (54 boys).

RESULTS

The 10th, 50th, and 90th percentiles of protein intake were 2.4, 2.9, and 4.0 g. kg(-1). d(-1), respectively; 63% was animal protein. In multiple linear regressions with adjustment for sex and weight, height (cm) was positively associated with intakes of animal protein (g/d) [0.10 +/- 0.038 (b +/- SE); P = 0.01] and milk (0.0047 +/- 0.002; P = 0.007), but not with those of vegetable protein or meat. The sIGF-I concentration was significantly associated with intakes of animal protein (1.4 +/- 0.53; P = 0.01) and milk (0.049 +/- 0.024; P = 0.045), but not with those of vegetable protein or meat. sIGF-I concentrations were positively associated with height (0.019 +/- 0.008; P = 0.02).

CONCLUSION

Milk intake was positively associated with sIGF-I concentrations and height. An increase in milk intake from 200 to 600 mL/d corresponded to a 30% increase in circulating IGF-I. This suggests that milk compounds have a stimulating effect on sIGF-I concentrations and, thereby, on growth.

Shift work, duration of pregnancy, and birth weight: the National Birth Cohort in Denmark

OBJECTIVE

We used the Danish National Birth Cohort to estimate the effect of shift work on the duration of pregnancy and birth weight.

STUDY DESIGN

In the Danish National Birth Cohort (1998-2001), we identified women (and their singleton births): 32465 women with daytime work, 1038 women with fixed evening work, 400 women with fixed night work, 3137 women with rotating shift work (without night), and 3197 women with rotating shift work (with night) during pregnancy (first and second trimester). Birth outcomes were collected by linkage to the Central Population Register, the National Patient Register, and the National Birth Register. Gestational age at birth and birth weight were analyzed by means of general linear regression and logistic regression.

RESULTS

Mean gestational age at birth and birth weight at term for daytime work were 281 days and 3616 g, although 281 days and 3586 to 3626 g for shift work. There were no statistically significant differences in gestational age at birth or birth weight at term between any types of shift work and daytime work. Fixed night work had a high risk of postterm birth (odds ratio, 1.35; 95% CI, 1.01-1.79); fixed evening work had a high risk of full-term low birth weight (odds ratio, 1.80; 95% CI, 1.10-2.94); and shift work as a group showed a slight excess of small-for-gestational-age babies (odds ratio, 1.09; 95% CI, 1.00-1.18). The risk of postterm birth was modified by maternal occupation. Industrial workers with fixed night work had a high risk of postterm birth.

CONCLUSION

Our results suggest that shift work during pregnancy had limited effects on indicators of fetal growth. Night work may prolong the duration of pregnancy and reduce fetal growth, especially among industrial workers.

HLA-G genotype is associated with fetoplacental growth

The human leukocyte antigen (HLA)-G is expressed by extravillous cytotrophoblast cells in the feto-maternal contact zone. Polymorphisms have been described in the HLA-G gene and have been linked with differences in HLA-G mRNA alternative splicing patterns and protein expression. Differences in the isoform profile or the degree of HLA-G expression may influence cytokine production and, thereby, placental and fetal growth. Associations between a 14 bp deletion polymorphism in the 3'UTR part of the HLA-G gene and birth weight in relation to gestational age and placental weight were studied in 47 pregnancies complicated with preeclampsia and 87 with no preeclampsia. An HLA-G genotype homozygous for the presence of the 14 bp sequence polymorphism was significantly associated with increased birth weight in relation to gestational age (one-way analysis of variance; 2 degrees of freedom: p = 0.02) and with placental weight at birth (>38 weeks of gestation; +14 bp/+14 bp vs others; unpaired t-test: p = 0.03). There was also a slightly higher placental ratio in the offspring with the +14 bp/+14 bp genotype. The implications of these findings are discussed in relation to certain complications of pregnancy and in an evolutionary perspective.

Diet and blood pressure in 2.5-y-old Danish children

BACKGROUND

Blood pressure tracks from childhood into adulthood, and early diet may have long-term effects on hypertension.

OBJECTIVE

The study's aim was to investigate whether intakes of n-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs) during lactation and current intakes of macronutrients affect blood pressure in 2.5-y-old Danish children.

DESIGN

Mothers (n =122) with low fish intakes were randomly assigned to receive supplementation with 4.5 g fish oil or olive oil/d during the first 4 mo of lactation. The trial also included 53 mothers with high fish intakes. One hundred five of these women's children attended a 2.5-y follow-up examination at which anthropometric data and blood pressure were obtained. Mothers then kept a 7-d dietary record of food consumed by their children. A full set of data from 73 children was analyzed for effects of fish oil supplementation and cross-sectional correlations with current diet.

RESULTS

We found no significant effect of the mothers' fish oil intakes during the first 4 mo of lactation on the blood pressure of the children 2.5 y later. Greater protein intakes measured as a percentage of energy were associated cross-sectionally with significantly lower diastolic and systolic blood pressures in the children at age 2.5 y after control for outdoor temperature, age, sex, weight, and height (P = 0.028 and 0.035, respectively). Greater protein intakes measured as g/d were also associated with significantly lower systolic blood pressures (P = 0.008). A 1-SD increase in protein intake corresponded with a decrease of approximately 3 mm Hg in systolic blood pressure.

CONCLUSION

The blood pressure of young Danish children was not significantly affected by intakes of n-3 LC-PUFAs via breast milk, but greater protein intakes at 2.5 y were associated with lower blood pressure.

Cohort study of sibling effect, infectious diseases, and risk of atopic dermatitis during first 18 months of life

OBJECTIVES

To determine whether early infectious diseases could explain the association between number of siblings and other markers of microbial exposure and the development of atopic dermatitis before the age of 18 months.

DESIGN

Cohort study. Information on atopic dermatitis, infectious diseases occurring before 6 months of age, number of siblings, early day care, pet keeping, farm residence, and background factors was collected in telephone interviews.

SETTING

Danish national birth cohort.

PARTICIPANTS

24,341 mother-child pairs.

MAIN OUTCOME MEASURES

Incidence rate ratios of atopic dermatitis.

RESULTS

13,070 children (54%) had at least one clinically apparent infectious disease before 6 months of age. At age 18 months, 2638 (10.8%) of the children had had atopic dermatitis. The risk of atopic dermatitis increased with each infectious disease before 6 months of age (incidence rate ratio 1.08, 95% confidence interval 1.04 to 1.13). The risk of atopic dermatitis decreased with each additional exposure to three or more siblings, day care, pet ownership, and farm residence (0.86, 0.81 to 0.93).

CONCLUSIONS

Early infections do not seem to protect against allergic diseases. The protective effect of number of siblings, day care, pet ownership, and farm residence remained after adjustment for clinically apparent infectious diseases, suggesting that the effect is established independently early in life.

Influence of maternal serum levels of vitamins C and E during the second trimester on birth weight and length

OBJECTIVE

It has been known that maternal oxidative stress during pregnancy plays an important role in fetal growth. However, the association between antioxidant vitamin levels and birth outcomes is not conclusive. We investigated the relationship between maternal serum levels of vitamins C and E during the second trimester and birth weight and length.

DESIGN

Prospective cohort study.

SETTING

Outpatient-clinic of obstetrics, Ewha Womans University Hospital, South Korea.

SUBJECTS AND METHODS

The study subjects were 239 healthy, pregnant women who visited an obstetric clinic for antenatal care, and their singleton live births, in Seoul, Korea, between August 2001 and March 2003. We measured the levels of vitamins C and E in maternal serum during the period 24-28 gestational weeks. Each woman was interviewed for dietary intake by trained interviewers during the second trimester.

RESULTS

The serum concentration of maternal vitamin C during the second trimester was significantly associated with birth weight and length in the group of full-term deliveries. An increase of 1 microg/ml in the serum vitamin C level increased the birth weight by 27.2 g and the birth length by 0.17 cm. When we considered the levels of vitamins C and E together in the relationship with birth weight and length, we found that the heaviest birth weight and the longest birth length belonged to the group of upper vitamin C/upper vitamin E. However, dietary intake estimated by 24-h recall method was not a predictor of the levels of serum vitamins C and E.

CONCLUSION

We found that maternal serum vitamin C levels during the second trimester were positively correlated with birth weight and length in full-term babies. We also found that birth weight and length were highest when the levels of both vitamins C and E were high. Our results indicate the importance of antioxidant nutrient balance for pregnant women who are exposed to various oxidants through food, drinking water, or inhaled air.