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

Supplementation with long chain polyunsaturated fatty acids (LCPUFA) to breastfeeding mothers for improving child growth and development

BACKGROUND

Long chain polyunsaturated fatty acids (LCPUFA), especially docosahexaenoic acid (DHA), are the most abundant fatty acids in the brain and are necessary for growth and maturation of the brain and retina. LCPUFA are named "essential" because they cannot be synthesised efficiently by the human body and come from maternal diet. It remains controversial whether LCPUFA supplementation to breastfeeding mothers is beneficial for the development of their infants.

OBJECTIVES

To assess the effectiveness and safety of supplementation with LCPUFA in breastfeeding mothers in the cognitive and physical development of their infants as well as safety for the mother and infant.

SEARCH STRATEGY

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (November 2009), CENTRAL (2009, Issue 2), PubMed (1966 to July 2009), EMBASE (1974 to June 2009), CINAHL (1984 to June 2009), LILACS (1982 to June 2009), Google Scholar (June 2009) and reference lists of published narrative and systematic reviews.

SELECTION CRITERIA

Randomised controlled trials or cluster-randomised controlled trials evaluating the effects of LCPUFA supplementation on breastfeeding mothers and their infants.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed eligibility and trial quality and performed data extraction.

MAIN RESULTS

We included six randomised controlled trials involving 1280 women. We found no significant difference in children's neurodevelopment: language development (standardised mean difference (SMD) -0.14, 95% confidence interval (CI) -0.49 to 0.20; two trials, 349 participants); intelligence or problem-solving ability (two trials, 817 participants; SMD -0.22, 95% CI -0.23 to 0.66); psychomotor development (SMD 0.34, 95% CI -0.11 to 0.78; two trials, 279 participants); motor development (SMD 0.08, 95% CI -0.13 to 0.29; two trials, 349 participants); in child attention there was a significant difference (SMD 0.50, 95% CI 0.24 to 0.77; one study). For child visual acuity there was no significant difference (SMD -0.06, 95% CI -0.26 to 0.14; three trials, 401 participants). For growth, there were significant differences in length (MD -0.75 cm, 95% CI -1.38 to -0.12; two trials, 834 participants) and head circumference (MD 0.69 cm, 95% CI 0.35 to 1.02; one trial, 244 participants). One study reported a significant difference in infant allergy (risk ratio (RR) 0.12, 95% CI 0.02 to 0.95). We found no significant difference in one trial evaluating postpartum depression (SMD 0.15, 95% CI -0.11 to 0.41).  

AUTHORS' CONCLUSIONS

Based on the limited evidence that we found, LCPUFA supplementation did not appear to improve children's neurodevelopment or visual acuity. In two studies, LCPUFA supplementation was associated with increased head circumference. Currently, there is insufficient evidence to support or refute the practice of giving LCPUFA supplementation to breastfeeding mothers in order to improve infant growth and development.

Prenatal exposures to perfluorinated chemicals and anthropometric measures in infancy

Perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) are persistent chemicals that may affect growth early in life. The authors estimated the associations between maternal plasma levels of PFOS and PFOA and infants' weight, length, and body mass index development during the first year of life. Fourteen hundred women were randomly selected from the Danish National Birth Cohort among those who provided blood samples early in pregnancy and gave birth to liveborn singletons between 1996 and 2002. Weight and length information at 5 and 12 months of age was available for 1,010 children. Multiple linear regression models were used for analyses, and maternal PFOS and PFOA concentrations (ng/mL) were inversely related to children's weight in the first year of life: adjusted regression coefficients: 0.8 g (95% confidence interval(CI): 4.2, 2.6) at 5 months and 5.8 g (95% CI:10.4, 1.2) at 12 months for perfluorooctanesulfonate(PFOS); 9.4 g (95% CI: 28.6, 9.9) at 5 months and 19.0 g (95% CI: 44.9, 6.8) at 12 months for perfluorooctanoate(PFOA) [corrected]. A similar pattern was observed for body mass index measurements, and no associations with length were found. After sex stratification, the inverse associations with weight and body mass index were more pronounced in boys, and no clear association was seen for girls.

Has fertility declined in recent decades?

OBJECTIVE

Increasing use of infertility treatment and a decline in demographic fertility in some countries have raised concern whether human fecundity is declining or has declined over time. A dramatic decline in semen quality over the past half-century has been proposed and widely discussed, but none of the existing studies provide data good enough for coming to any firm conclusion.

RESULTS

Results from 19 articles published during the last 5 years on semen quality and fecundity, covering data from the Nordic countries since the year 2000, do not indicate an ongoing decline in fertility.

CONCLUSION

We will probably never know, whether semen quality and fecundity has declined over time, but we may be able to monitor biological fecundity and semen quality over time from now on. There are both research initiatives and interest from official channels that could provide more funding for infertility research.

Use of proton-pump inhibitors in early pregnancy and the risk of birth defects

BACKGROUND

Symptoms of gastroesophageal reflux are common in pregnancy, but there are limited data on the risk of birth defects associated with exposure to proton-pump inhibitors (PPIs) in early pregnancy.

METHODS

We conducted a cohort study to assess the association between exposure to PPIs during pregnancy and the risk of major birth defects among all infants born alive in Denmark between January 1996 and September 2008. We linked data from nationwide registries, including individual-level information on exposure to PPIs (prescriptions), birth defects, and potential confounders. Major birth defects, diagnosed within the first year of life, were categorized according to the standardized classification scheme of the European surveillance of congenital anomalies (EUROCAT). Our primary analyses assessed the use of PPIs from 4 weeks before conception through 12 weeks of gestation and from 0 through 12 weeks of gestation (first trimester).

RESULTS

Among 840,968 live births, 5082 involved exposure to PPIs between 4 weeks before conception and the end of the first trimester of pregnancy. There were 174 major birth defects in infants whose mothers had been exposed to PPIs during this period (3.4%), as compared with 21,811 in the group whose mothers had not been exposed (2.6%) (adjusted prevalence odds ratio, 1.23; 95% confidence interval [CI], 1.05 to 1.44). In analyses limited to exposure during the first trimester, there were 118 major birth defects among 3651 infants exposed to PPIs (3.2%), and the adjusted prevalence odds ratio was 1.10 (95% CI, 0.91 to 1.34). The risk of birth defects was not significantly increased in secondary analyses of exposure to individual PPIs during the first trimester or in analyses limited to the offspring of women who had filled PPI prescriptions and received enough doses to have a theoretical chance of first-trimester exposure.

CONCLUSIONS

In this large cohort, exposure to PPIs during the first trimester of pregnancy was not associated with a significantly increased risk of major birth defects. (Funded by the Danish Medical Research Council and the Lundbeck Foundation.).

Risk of infection and adverse outcomes among pregnant working women in selected occupational groups: A study in the Danish National Birth Cohort

BACKGROUND

Exposure to infectious pathogens is a frequent occupational hazard for women who work with patients, children, animals or animal products. The purpose of the present study is to investigate if women working in occupations where exposure to infections agents is common have a high risk of infections and adverse pregnancy outcomes.

METHODS

We used data from the Danish National Birth Cohort, a population-based cohort study and studied the risk of Infection and adverse outcomes in pregnant women working with patients, with children, with food products or with animals. The regression analysis were adjusted for the following covariates: maternal age, parity, history of miscarriage, socio-occupational status, pre-pregnancy body mass index, smoking habit, alcohol consumption.

RESULTS

Pregnant women who worked with patients or children or food products had an excess risk of sick leave during pregnancy for more than three days. Most of negative reproductive outcomes were not increased in these occupations but the prevalence of congenital anomalies (CAs) was slightly higher in children of women who worked with patients. The prevalence of small for gestational age infants was higher among women who worked with food products. There was no association between occupation infections during pregnancy and the risk of reproductive failures in the exposed groups. However, the prevalence of CAs was slightly higher among children of women who suffered some infection during pregnancy but the numbers were small.

CONCLUSION

Despite preventive strategies, working in specific jobs during pregnancy may impose a higher risk of infections, and working in some of these occupations may impose a slightly higher risk of CAs in their offspring. Most other reproductive failures were not increased in these occupations.

Intrauterine exposure to mild analgesics is a risk factor for development of male reproductive disorders in human and rat

BACKGROUND

More than half of pregnant women in the Western world report intake of mild analgesics, and some of these drugs have been associated with anti-androgenic effects in animal experiments. Intrauterine exposure to anti-androgens is suspected to contribute to the recent increase in male reproductive problems, and many of the anti-androgenic compounds are like the mild analgesics potent inhibitors of prostaglandin synthesis. Therefore, it appears imperative to further investigate the potential endocrine disrupting properties of mild analgesics.

METHODS

In a prospective birth cohort study, 2297 Danish and Finnish pregnant women completed a questionnaire and 491 of the Danish mothers participated in a telephone interview, reporting on their use of mild analgesics during pregnancy. The testicular position of newborns was assessed by trained paediatricians. In rats, the impact of mild analgesics on anogenital distance (AGD) after intrauterine exposure was examined together with the effect on ex vivo gestational day 14.5 testes.

RESULTS

In the Danish birth cohort, the use of mild analgesics was dose-dependently associated with congenital cryptorchidism. In particular, use during the second trimester increased the risk. This risk was further increased after the simultaneous use of different analgesics. The association was not found in the Finnish birth cohort. Intrauterine exposure of rats to paracetamol led to a reduction in the AGD and mild analgesics accordingly reduced testosterone production in ex vivo fetal rat testes.

CONCLUSION

There was an association between the timing and the duration of mild analgesic use during pregnancy and the risk of cryptorchidism. These findings were supported by anti-androgenic effects in rat models leading to impaired masculinization. Our results suggest that intrauterine exposure to mild analgesics is a risk factor for development of male reproductive disorders.

Circulating predictive biomarkers in preeclampsia

The aim of the paper was to review promising prediction biomarkers for preeclampsia. The methods included review of abstracts on PubMed, using "preeclampsia", "biomarker", "prediction" as key words, followed by review of primary papers investigating blood based biomarkers. Angiogenic factors, such as PlGf and sFlt1, and PP13 seem presently to have the best predictive test values for preeclampsia, but sensitivity and specificity is still too low to prove useful in a population screening setting. Biomarker testing should still be part of research protocols. Biomarkers could provide useful in selecting high-risk women for prophylactic trials as well as identifying pregnancies that could profit from closer follow-up.

Parental infertility and cerebral palsy in children

BACKGROUND

Children born after in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) have been reported to have a higher risk of cerebral palsy (CP), perhaps due to the higher frequency of preterm birth, multiple births or vanishing embryo in the pregnancies. However, it has been suggested that the underlying infertility may be part of the pathway. In this study, we examined whether untreated subfecundity (measured by time to pregnancy) or infertility treatment was associated with an increased risk of CP in the offspring.

METHODS

Using the Danish National Birth Cohort (1997-2003), we compared children born after 0-2 months of waiting time to pregnancy (n = 35 848) with those born after a time to pregnancy of 3-5 months (n = 15 361), 6-12 months (n = 11 528) and >12 months (n = 7387), as well as those born after IVF/ICSI (n = 3617), ovulation induction with or without intrauterine insemination (n = 3000), and unplanned pregnancies (n = 13 462). CP cases were identified through the Danish CP Register.

RESULTS

In total, 165 (0.18%) children were diagnosed with CP in the entire cohort. We found no significant association between time to pregnancy and the risk of CP in children conceived spontaneously. Children born after IVF/ICSI had an increased risk of CP, even after adjustment for preterm birth and multiplicity (hazard ratio 2.30, 95% confidence interval 1.12-4.73).

CONCLUSIONS

Subfecundity per se did not appear to be associated with the risk of CP in children, whereas being born after IVF/ICSI conferred an increased risk.

Prenatal exposure to PFOA and PFOS and risk of hospitalization for infectious diseases in early childhood

OBJECTIVES

To examine whether prenatal exposure to perfluorooctanesulfonate (PFOS) or perfluorooctanoate (PFOA) is associated with the occurrence of hospitalization for infectious diseases during early childhood.

METHODS

We randomly selected 1400 pregnant women and their offspring from the Danish National Birth Cohort (1996-2002) and measured PFOS and PFOA levels in maternal blood during early pregnancy. Hospitalizations for infection of the offspring were identified by the linkage to the National Hospital Discharge Register through 2008.

RESULTS

Hospitalizations due to infections were not associated with prenatal exposure to PFOA and PFOS. On the contrary, the relative risks of hospitalizations ranged from 0.71 to 0.84 for the three higher quartiles of maternal PFOA levels compared with the lowest, but no dose-response pattern was found. No clear pattern was observed when results were stratified by child's age at infection, with the exception of an inverse association between maternal PFC levels and risk of hospitalization during the child's first year of life.

CONCLUSIONS

These findings suggest that prenatal exposure to PFOA or PFOS is not associated with increased risk of infectious diseases leading to hospitalization in early childhood.

Early influences on cardiovascular and renal development

The hypothesis that a developmental component plays a role in subsequent disease initially arose from epidemiological studies relating birth size to both risk factors for cardiovascular disease and actual cardiovascular disease prevalence in later life. The findings that small size at birth is associated with an increased risk of cardiovascular disease have led to concerns about the effect size and the causality of the associations. However, recent studies have overcome most methodological flaws and suggested small effect sizes for these associations for the individual, but an potential important effect size on a population level. Various mechanisms underlying these associations have been hypothesized, including fetal undernutrition, genetic susceptibility and postnatal accelerated growth. The specific adverse exposures in fetal and early postnatal life leading to cardiovascular disease in adult life are not yet fully understood. Current studies suggest that both environmental and genetic factors in various periods of life may underlie the complex associations of fetal growth retardation and low birth weight with cardiovascular disease in later life. To estimate the population effect size and to identify the underlying mechanisms, well-designed epidemiological studies are needed. This review is focused on specific adverse fetal exposures, cardiovascular adaptations and perspectives for new studies.

Smoking and pregnancy-related pelvic pain

OBJECTIVE

To investigate possible associations between smoking and pregnancy-related pelvic pain.

DESIGN

Nested case-control study.

SETTING

Denmark 2000-2001.

POPULATION

The Danish National Birth Cohort.

METHODS

The women were interviewed twice in pregnancy and twice after childbirth. The first pregnancy interview provided information on smoking and possible confounding factors,whereas the first interview after birth addressed case identification.Cases (n = 2302) were defined on the basis of self-reported pelvic pain, and controls were selected among women who did not report pelvic pain (n = 2692). Logistic regression analysis was used to estimate associations between smoking and pelvic pain.

MAIN OUTCOME MEASURE

Pregnancy-related pelvic pain.

RESULTS

Compared with non-smokers, women who smoked during pregnancy had an adjusted odds ratio of 1.2 (1.0-1.4) for overall pelvic pain, similar to women who stopped smoking in early pregnancy 1.3 (1.1-1.7). The equivalent adjusted odds ratio for severe pelvic pain was 1.2 (1.0-1.5) for smokers, and 1.5 (1.2-1.9)for women who stopped smoking. Smoking intensity, measured as number of cigarettes smoked per day, was associated with pelvic pain in a dose-response pattern. Information about smoking was collected prospectively, which makes it unlikely that differential recall alone explains the results.

CONCLUSIONS

Smoking was associated with pregnancy-related pelvic pain, with a dose-response pattern between reported smoking intensity and pelvic pain. These findings suggest a possible new risk factor for a common ailment during pregnancy

Intake of artificially sweetened soft drinks and risk of preterm delivery: a prospective cohort study in 59,334 Danish pregnant women

BACKGROUND

Sugar-sweetened soft drinks have been linked to a number of adverse health outcomes such as high weight gain. Therefore, artificially sweetened soft drinks are often promoted as an alternative. However, the safety of artificial sweeteners has been disputed, and consequences of high intakes of artificial sweeteners for pregnant women have been minimally addressed.

OBJECTIVE

We examined the association between intakes of sugar-sweetened and artificially sweetened soft drinks and preterm delivery.

DESIGN

We conducted prospective cohort analyses of 59,334 women from the Danish National Birth Cohort (1996-2002). Soft drink intake was assessed in midpregnancy by using a food-frequency questionnaire. Preterm delivery ( lt 37 wk) was the primary outcome measure. Covariate information was assessed by telephone interviews.

RESULTS

There was an association between intake of artificially sweetened carbonated and noncarbonated soft drinks and an increased risk of preterm delivery (P for trend: le 0.001, both variables). In comparison with women with no intake of artificially sweetened carbonated soft drinks, the adjusted odds ratio for women who consumed ge 1 serving of artificially sweetened carbonated soft drinks/d was 1.38 (95% CI: 1.15, 1.65). The corresponding odds ratio for women who consumed ge 4 servings of artificially sweetened carbonated soft drinks/d was 1.78 (95% CI: 1.19, 2.66). The association was observed for normal-weight and overweight women. A stronger increase in risk was observed for early preterm and moderately preterm delivery than with late-preterm delivery. No association was observed for sugar-sweetened carbonated soft drinks (P for trend: 0.29) or for sugar-sweetened noncarbonated soft drinks (P for trend: 0.93).

CONCLUSIONS

Daily intake of artificially sweetened soft drinks may increase the risk of preterm delivery. Further studies are needed to reject or confirm these findings.

Use of acetaminophen during pregnancy and risk of preeclampsia, hypertensive and vascular disorders: a birth cohort study

OBJECTIVE

To examine whether pregnant women who used acetaminophen, a prostaglandinG2 synthase inhibitor, had an increased risk of preeclampsia, gestational hypertension, thromboembolic complications, or abruptio placentae.

METHODS

We selected 63,833 women participating in the Danish National Birth Cohort who gave birth to a live born singleton and had information on acetaminophen use during pregnancy reported by three interviews. Through linkage to the National Hospital Discharge Registry we obtained data from hospital diagnose of the outcomes we study.

RESULTS

Women who used acetaminophen during the third trimester of pregnancy had an increased risk of preeclampsia (adjusted relative risk RR = 1.40, 95% CI: 1.24-1.58). The risk was higher among women who had early preeclampsia (before the 32nd gestational week) (RR = 1.47, 95% CI: 1.12-1.93), severe preeclampsia (RR = 1.51, 95% CI: 1.15-2.00), or chronic hypertension (RR = 1.44, 95% CI: 1.13-1.83). Second and third trimester use was associated with an increased risk of pulmonary embolisms (RR = 3.02, 1.28-7.15) and deep vein thrombosis (RR = 2.15, 1.06-4.37), respectively.

CONCLUSIONS

Acetaminophen use during pregnancy is associated with an increased risk of diseases in which a reduction of prostacyclin during pregnancy has been postulated to play a role, including preeclampsia and thromboembolic diseases.

Maternal postpartum distress and childhood overweight

OBJECTIVE

We investigated associations between maternal postpartum distress covering anxiety, depression and stress and childhood overweight.

METHODS

We performed a prospective cohort study, including 21,121 mother-child-dyads from the Danish National Birth Cohort (DNBC). Maternal distress was measured 6 months postpartum by 9 items covering anxiety, depression and stress. Outcome was childhood overweight at 7-years-of age. Multiple logistic regression analyses were performed and information on maternal age, socioeconomic status, pre-pregnancy BMI, gestational weight gain, parity, smoking during pregnancy, paternal BMI, birth weight, gestational age at birth, sex, breastfeeding and finally infant weight at 5 and 12 month were included in the analyses.

RESULTS

We found, that postpartum distress was not associated with childhood risk of overweight, OR 1.00, 95%CI [0.98-1.02]. Neither was anxiety, depression, or stress exposure, separately. There were no significant differences between the genders. Adjustment for potential confounders did not alter the results.

CONCLUSION

Maternal postpartum distress is apparently not an independent risk factor for childhood overweight at 7-years-of-age. However, we can confirm previous findings of perinatal determinants as high maternal pre-pregnancy BMI, and smoking during pregnancy being risk factors for childhood overweight.

Distribution and predictors of exercise habits among pregnant women in the Danish National Birth Cohort

Physical activity is recommended during pregnancy, although strong evidence on reproductive health is lacking. We present exercise habits and predictors of exercise during pregnancy. From the Danish National Birth Cohort (1996-2002), 88,200 singleton pregnancies were analyzed in logistic regression. About one-third of the women exercised in early/mid pregnancy and slightly less in late pregnancy. Bicycling, swimming, and low-impact activities were most common. Exercising more than three times per week was strongly correlated with older age, being a student or out of work, eating disorders, moderate alcohol consumption, and a healthy diet. Multiparity, a normal or less good self-rated health, smoking, and a less health conscious diet were the strongest predictors of not doing exercise. Women of 25 years or older, with metabolic or psychiatric disorders, or who had received subfecundity treatment were more likely to increase their activity level substantially from early to late pregnancy than comparison groups. In conclusion, exercising during pregnancy correlated with a number of maternal characteristics. The findings may be used to identify pregnant women not likely to exercise, to target activities that may fit their needs, and, for research purposes, to identify adjustment variables or guide sensitivity analyses when data on confounders are lacking.

Polish mother and child cohort study--defining the problem, the aim of the study and methodological assumption

OBJECTIVES

Exposures during prenatal period have implications for pregnancy outcome as well as for children's health, morbidity and mortality. Prospective cohort study design allows for the identification of exposures that may influence pregnancy outcome and children's health, verification of such exposures by biomarker measurements and notification of any changes in exposure level.

MATERIALS AND METHODS

Polish Mother and Child Cohort Study (REPRO_PL) is multicenter prospective cohort study conducted in 8 different regions of Poland. The final cohort is intended to comprise 1300 mother-child pairs to be recruited within 4-year period (2007-2011). The recruitment and all scheduled visits are conducted in maternity units or clinics in the districts included in the study. The women are followed-up 3 times in pregnancy (once in each trimester) and after delivery for the notification of pregnancy outcome. During each visit, detailed questionnaire and biological samples are collected including saliva, urine, hair, maternal blood and cord blood. About 6 weeks postpartum, breast milk from part of the women is collected. The study concentrates on the identification and evaluation of the effects of prenatal environmental exposure on pregnancy outcome and children's health. Specific research hypotheses refer to the role of heavy metals, exposure to polycyclic aromatic hydrocarbons (PAHs) and environmental tobacco smoke (ETS) in the aetiology of small-for-gestational-age (SGA) and preterm delivery (PD). The role of oxidative stress putative mechanism and pregnant women nutritional status will be investigated. Based on questionnaire data, the impact of occupational exposures and stressful situations will be evaluated.

RESULTS

The results of the study will become available within the next few years and will help to determine levels of child prenatal exposure in several areas of Poland and its impact on course and outcome of pregnancy.

Non-occupational exposure to paint fumes during pregnancy and fetal growth in a general population

Occupational exposure to organic solvents during pregnancy has been associated with reduced fetal growth. Though organic solvents in the form of paint fumes are also found in the home environment, no studies have investigated the effect of such exposure in a general population. We studied associations between residential exposure to paint fumes during pregnancy and fetal growth within the Danish National Birth Cohort which consecutively recruited pregnant women from 1996 to 2002 from all over Denmark. Around the 30th pregnancy week, 19,000 mothers were interviewed about use of paint in their residence during pregnancy. The mothers were also asked about smoking habits and alcohol consumption during pregnancy, pre-pregnancy weight, height, parity and occupation. Information on birth weight and gestational age was obtained from national registers. We found that 45% of the mothers had been exposed to paint fumes in their residence during pregnancy. We found a statistically significant inverse relationship between exposure to paint fumes and the risk of being small for gestational age. There were no statistically significant associations between exposure to paint fumes and birth weight and risk of preterm birth after adjustment for potential confounders. Our results suggest that there are no causal relationship between non-occupational exposure to paint fumes in the residence during pregnancy and fetal growth.

Maternal psychosocial adversity during pregnancy is associated with length of gestation and offspring size at birth: evidence from a population-based cohort study

OBJECTIVE

To study in a large-scale cohort with prospective data the associations of psychosocial adversities during pregnancy with length of gestation and offspring size at birth.

METHODS

We defined a priori two types of psychosocial adversity during pregnancy: life stress (perceived burdens in major areas of life) and emotional symptoms (e.g. anxiety). Measures of offspring size at birth, including body weight, body length, abdominal and head circumference, were obtained from a national medical birth registry. We included in the analyses gestational age and offspring size at birth controlled for length of gestation; the latter was calculated by gestational-age-specific z scores (ZS) reported in 10(-3). We conducted multiple regression analyses adjusted for potential confounders to estimate the association between exposures and birth outcomes (n = 78017 pregnancies).

RESULTS

Life stress (per score increase by 1; range, 0-18) was associated with shorter length of gestation (days; B, -0.14; 95% confidence interval (CI), -0.19, -0.10), increased offspring body weight (ZS; B, 9.14; 95% CI, 4.99, 13.28), body length (ZS; B, 6.58; 95% CI, 2.39, 10.77), abdominal circumference (ZS; B, 9.96; 95% CI, 5.77, 14.16), and head circumference (ZS; B, 6.13; 95% CI, 1.95, 10.30). Emotional symptoms were associated with shorter length of gestation (days; B, -0.04; 95% CI, -0.07, -0.004) and decreased body length (ZS; B, -4.44; 95% CI, -7.57, -1.32) only.

CONCLUSIONS

Life stress and emotional symptoms both predicted a shorter length of gestation, while only life stress predicted an increased offspring size at birth controlled for length of gestation; yet, the associations were rather small. The fetoplacental-maternal unit may regulate fetal growth according to the type of psychosocial adversity and even increase fetal growth in response to maternal stress in major areas of life. This potentially reflects a basic principle of intrauterine human development in response to stress.

Maternal use of nicotine replacement therapy during pregnancy and offspring birthweight: a study within the Danish National Birth Cohort

Smoking is a well-established risk factor for fetal growth restriction and other adverse pregnancy outcomes, and nicotine may be one of the chemical compounds that drive these associations. Nicotine replacement therapy (NRT) is a smoking cessation aid, which can facilitate smoking cessation. It is, however, unknown whether NRT used during pregnancy impairs fetal growth. The aim of this study was to estimate the association between the use of NRT during pregnancy and offspring birthweight. The study population consisted of 72 761 women enrolled in the Danish National Birth Cohort between 1996 and 2002. Information on NRT and potential confounders was obtained from two computer-assisted telephone interviews conducted in the second and third trimesters, respectively. Multiple linear regression in a multilevel model was used to estimate the association between NRT use and birthweight adjusted for gestational age and potential confounders. The adjusted analyses showed no significant association between the duration of NRT use and birthweight (b = 0.25 g per week of NRT use [95% CI -2.31, 2.81]) and neither was the type of NRT product (patch, gum, inhaler) associated with reduced birthweight. However, simultaneous use of more than one NRT product was associated with reduced birthweight (b = -10.73 g per week of NRT use [95% CI -26.51, 5.05]), although the association was not statistically significant. The results of this study suggest that maternal use of NRT in pregnancy does not seriously affect birthweight, but there could be a negative effect on birthweight associated with simultaneous use of more than one type of NRT product.

A successful implementation of e-epidemiology: the Danish pregnancy planning study 'Snart-Gravid'

The attraction of being able to use the internet for the recruitment of an epidemiologic cohort stems mainly from cost efficiency and convenience. The pregnancy planning study ('Snart-Gravid')-a prospective cohort study of Danish women planning a pregnancy-was conducted to evaluate the feasibility and cost efficiency of using internet-based recruitment and follow-up. Feasibility was assessed by examining patient accrual data over time, questionnaire-specific response rates and losses to follow-up. The relative cost efficiency was examined by comparing the study costs with those of an alternative non internet-based study approach. The target recruitment of 2,500 participants over 6 months was achieved using advertisements on a health-related website, supported by a coordinated media strategy at study initiation. Questionnaire cycle-specific response rates ranged from 87 to 90% over the 12-month follow-up. At 6 months, 87% of women had a known outcome or were still under follow-up; at 12 months the figure was 82%. The study cost of $400,000 ($160 per enrolled subject) compared favorably with the estimated cost to conduct the same study using a conventional non-internet based approach ($322 per subject). The gain in efficiency with the internet-based approach appeared to be even more substantial with longer follow-up and larger study sizes. The successful conduct of this pilot study suggests that the internet may be a useful tool to recruit and follow subjects in prospective cohort studies.

In-utero exposure to smoking, alcohol, coffee, and tea and risk of strabismus

In a prospective, population-based cohort study, the authors investigated the effect of in-utero exposure to maternal smoking and consumption of alcohol, coffee, and tea on the risk of strabismus. They reviewed medical records for children in the Danish National Birth Cohort identified through national registers as possibly having strabismus. Relative risk estimates were adjusted for year of birth, social class, maternal smoking, maternal age at birth, and maternal coffee and tea consumption. The authors identified 1,321 cases of strabismus in a cohort of 96,842 Danish children born between 1996 and 2003. Maternal smoking was associated with a significantly elevated risk of strabismus in the child, increasing with number of cigarettes smoked per day (<5 cigarettes/day: relative risk (RR) = 0.95, 95% confidence interval (CI): 0.80, 1.14; 5-<10 cigarettes/day: RR = 1.38, 95% CI: 1.12, 1.70; > or =10 cigarettes/day: RR = 1.90, 95% CI: 1.57, 2.30). Nicotine replacement therapy was not associated with strabismus risk (RR = 1.22, 95% CI: 0.92, 1.61). Light maternal alcohol consumption was inversely associated with strabismus risk, whereas maternal coffee and tea drinking were not associated with strabismus risk. In conclusion, smoking during pregnancy is associated with an increased risk of strabismus in the offspring. Conversely, light alcohol consumption is associated with decreased risk.

Population based external validation of a European predictive model for respiratory syncytial virus hospitalization of premature infants born 33 to 35 weeks of gestational age

Prospectively collected population-based data on 2529 Danish infants born at 33 to 35 weeks of gestation were used to validate an European predictive model of respiratory syncytial virus (RSV) hospitalization. The model was found to be robust with a diagnostic accuracy of 65.9% to distinguish between RSV-hospitalized versus non-RSV-hospitalized Danish infants born at 33 to 35 weeks of gestation.

Selection by socioeconomic factors into the Danish National Birth Cohort

BACKGROUND

Low participation at recruitment to the Danish National Birth Cohort (DNBC) has raised concern about non-participation bias. Objective To study the socioeconomic pattern of participation to the DNBC.

METHODS

Independently of the DNBC, we identified the DNBC source population in two geographical areas of Denmark by means of local birth registers with full coverage. Socioeconomic information came from national registers, and the source population consisted of 48,560 births including 15,290 participating women. For every socioeconomic characteristic, we estimated the prevalence ratio [prevalence (participants)/prevalence (source population)] which corresponds to the relative representation of the group (presented in percentages with 95% confidence intervals).

RESULTS

The overall participation rate was 31%. Women outside the work force or with no further education than compulsory school were underrepresented in the DNBC by 62% (59%; 64%) and 43% (41%; 45%), respectively. Also, women were underrepresented by 18% (13%; 23%) if they were unemployed, by 22% (20%; 24%) if they were in the lowest income group, 38% (35%; 40%) if they received a high proportion of social benefits, and 28% (24%; 31%) if they were singles. Particularly women with low resources according to two socioeconomic factors were strongly underrepresented, typically by 50-67%.

CONCLUSION

Groups with low socioeconomic resources in terms of education, occupation, income and civil status are underrepresented in the DNBC compared to the background population. These discrepancies must be taken into account when results from the DNBC and other cohorts of pregnant women are interpreted-especially when descriptive results are presented.

Protocol of the baseline assessment for the Environments for Healthy Living (EHL) Wales cohort study

BACKGROUND

Health is a result of influences operating at multiple levels. For example, inadequate housing, poor educational attainment, and reduced access to health care are clustered together, and are all associated with reduced health. Policies which try to change individual people's behaviour have limited effect when people have little control over their environment. However, structural environmental change and an understanding of the way that influences interact with each other, has the potential to facilitate healthy choices irrespective of personal resources. The aim of Environments for Healthy Living (EHL) is to investigate the impact of gestational and postnatal environments on health, and to examine where structural change can be brought about to optimise health outcomes. The baseline assessment will focus on birth outcomes and maternal and infant health.

METHODS/DESIGN

EHL is a longitudinal birth cohort study. We aim to recruit 1000 pregnant women in the period April 2010 to March 2013. We will examine the impact of the gestational environment (maternal health) and the postnatal environment (housing and neighbourhood conditions) on subsequent health outcomes for the infants born to these women. Data collection will commence during the participants' pregnancy, from approximately 20 weeks gestation. Participants will complete a questionnaire, undergo anthropometric measurements, wear an accelerometer, compile a food diary, and have environmental measures taken within their home. They will also be asked to consent to having a sample of umbilical cord blood taken following delivery of their baby. These data will be complemented by routinely collected electronic data such as health records from GP surgeries, hospital admissions, and child health and development records. Thereafter, participants will be visited annually for follow-up of subsequent exposures and child health outcomes.

DISCUSSION

The baseline assessment of EHL will provide information concerning the impact of gestational and postnatal environments on birth outcomes and maternal and infant health. The findings can be used to inform the development of complex interventions targeted at structural, environmental factors, intended to reduce ill-health. Long-term follow-up of the cohort will focus on relationships between environmental exposures and the later development of adverse health outcomes, including obesity and diabetes.

Fetal exposure to antidepressants and normal milestone development at 6 and 19 months of age

OBJECTIVE

The goal was to investigate a possible association between exposure to antidepressants in utero and developmental milestones in early childhood.

METHODS

Information on pregnancy exposures and developmental milestones at 6 and 19 months was obtained from the Danish National Birth Cohort. Of the eligible pregnant women, 415 used antidepressant medication, 489 reported depression with no medical treatment, and 81 042 reported no depression and no use of psychotropic medication.

RESULTS

Children with second- or third-trimester exposure to antidepressants were able to sit 15.9 days (95% confidence interval [CI]: 6.8-25.0) and to walk 28.9 days (95% CI: 15.0-42.7) later than children of women not exposed to antidepressants but still were within the normal range of development. Fewer children with second- or third-trimester exposure to antidepressants were able to sit without support at 6 months of age (odds ratio: 2.1 [95% CI: 1.23-3.60]), and fewer were able to occupy themselves at 19 months of age (odds ratio: 2.1 [95% CI: 1.09-4.02]). None of the other milestones measured showed statistically significant associations with antidepressant exposure.

CONCLUSION

The results of this study suggest a permanent or reversible effect of antidepressant exposure on fetal brain development, which may depend on the timing of exposure during pregnancy.

Spontaneous labor onset: is it immunologically mediated?

OBJECTIVE

The investigators tested the hypothesis that maternal-fetal immune interactions could be important in initiating spontaneous labor onset by examining if labor was delayed when fetuses share maternal HLA antigen types.

STUDY DESIGN

HLA antigen types A, B, and DR in 200 Danish mother-infant pairs delivering in 42-44 weeks (postterm) were compared with 195 mother-infant pairs delivering in 37-40 weeks (term).

RESULTS

Sharing of HLA A and B antigens was more common than expected in postterm deliveries. Odds ratios were 1.54 (95% confidence interval [CI], 1.01-2.35) and 1.75 (95% CI, 0.87-3.52), respectively (risk per shared antigen: 1.40 [95% CI, 1.04-1.90] per unit increase). Adding stringent birth-length criteria for postmaturity (92 cases; 168 controls) strengthened risks associated with antigen sharing to 1.57 (95% CI, 0.90-2.74) and 2.60 (95% CI, 1.15-5.88), respectively (risk per shared antigen: 1.60 (95% CI, 1.10-2.32).

CONCLUSION

Postterm-delivered infants had more HLA A and B antigens in common with their mothers, suggesting that recognition of HLA antigen differences by adaptive immunity may have a role in triggering labor onset.

Mission in Sukusuku cohort, Mie: focusing on the feasibility and validity of methods for enrolling and retaining participants

Background

We investigated the feasibility and validity of and systematized the methods used to enroll and retain participants requiring long-term interdisciplinary collaborations. We carried out this study in the Sukusuku cohort, Mie (SCM), as one of the regional research site of Japan Children’s Study (JCS).

Methods

A total of 467 families who were screened between December 1, 2004 and December 31, 2005, in the Mie-chuo Medical Center and 2 other hospitals; these families were deemed eligible for the study. Of these, a total of 185 families (39.6%) participated in the 4-month observation. Of these families, 5 dropped out at month 9 of the observation; 9, at month 18; 17, at month 30; and 5, at month 42. The retention rates at 9, 18, 30, and 42 months of observation were 97.3%, 92.4%, 83.2%, and 80.5%, respectively. Reinstatement to a previous job was the most common reason for dropouts.

Results

We observed that informative consultation notes during observation were beneficial for the retention of participants, and these notes also helped in improving communication between the study subjects and the evaluators during subsequent visits.

Conclusions

In this study, we did not perform the standard checks for child development alone but also investigated the motivating influence of research partnerships with participants. Further, these visits help maintain the motivation levels of the participants and encourage them to contribute for social causes. The results present integration models that can be applied in future relevant longitudinal cohort studies in Japan.

Parental infertility and developmental coordination disorder in children

BACKGROUND

It has previously been reported that children born after infertility treatment had a slight delay in early motor milestones. In this study, we examined whether children of infertile couples with or without infertility treatment had a higher risk of developmental coordination disorder (DCD).

METHODS

We used data on parental infertility and DCD among 23 167 singletons from the Danish National Birth Cohort (1996-2002). Data on time to pregnancy (TTP) and infertility treatment were collected early in pregnancy. Data on DCD in children were collected using the Developmental Coordination Disorder Questionnaire, filled in by the mothers during follow-up when the children were 7 years old. We used the recommended cut-off for the age group to classify children.

RESULTS

Compared with children born of fertile couples, children conceived after a waiting TTP of longer than 12 months had a slightly higher risk of DCD [odds ratio (OR) 1.35, 95% confidence interval (CI) 1.03-1.77], but the estimated OR was not significant in children born after infertility treatment (OR 1.19, 95% CI 0.86-1.66). None of the individual treatment procedures was significantly associated with a higher risk of DCD. Children of parents who had not planned their pregnancy showed no elevated risk.

CONCLUSIONS

Our findings are overall reassuring, although it is possible that low fecundity may be associated with a modestly increased risk of DCD.

Large prospective birth cohort studies on environmental contaminants and child health - goals, challenges, limitations and needs

The adverse health effects of environmental contaminants (ECs) are a rising public health concern, and a major threat to sustainable socioeconomic development. The developing fetuses and growing children are particularly vulnerable to the adverse effects of ECs. However, assessing the health impact of ECs presents a major challenge, given that multiple outcomes may arise from one exposure, multiple exposures may result in one outcome, and the complex interactions between ECs, and between ECs, nutrients and genetic factors, and the dynamic temporal changes in EC exposures during the life course. Large-scale prospective birth cohort studies collecting extensive data and specimen starting from the prenatal or pre-conception period, although costly, hold promise as a means to more clearly quantify the health effects of ECs, and to unravel the complex interactions between ECs, nutrients and genotypes. A number of such large-scale studies have been launched in some developed counties. We present an overview of "why", "what" and "how" behind these efforts with an objective to uncover major unidentified limitations and needs. Three major limitations were identified: (1) limited data and bio-specimens regarding early life EC exposure assessments in some birth cohort studies; (2) heavy participant burdens in some birth cohort studies may bias participant recruitment, and risk substantial loss to follow-up, protocol deviations limiting the quality of data and specimens collection, with an overall potential bias towards the null effect; (3) lack of concerted efforts in building comparable birth cohorts across countries to take advantage of natural "experiments" (large EC exposure level differences between countries) for more in-depth assessments of dose-response relationships, threshold exposure levels, and positive and negative effect modifiers. Addressing these concerns in current or future large-scale birth cohort studies may help to produce better evidence on the health effects of ECs.

HLA antigen sharing between mother and fetus as a risk factor for eclampsia and preeclampsia

Immune maladaption between mother and infant has been suggested to induce preeclampsia/eclampsia. When fetuses share more human leukocyte antigen (HLA) types with their mother, immune differences would be limited and thereby could affect this risk. Data from Danish women (1996-2002) with single live-birth pregnancies complicated by severe preeclampsia/eclampsia were compared to women with term pregnancies uncomplicated by hypertension. HLA A, B, and DR types were resolved at the intermediate-level typing (antigen). A total of 201 cases and 195 control mother-infant pairs had complete HLA types. The odds ratios of preeclampsia/eclampsia in mothers sharing both HLA antigens with their infants were 1.19 (95% confidence interval: 0.81-1.76) for HLA A, 0.91 (0.59-1.42) for HLA B, and 1.05 (0.5-1.59) for HLA DR antigens. No specific HLA antigens in either mother or infant appeared important after Bonferroni correction, except possibly DR01 in mothers (protective). Thus, maladaption mediated by adaptive immunity between mother and infant is not the basis for the mother developing preeclampsia/eclampsia.

Physical exercise during pregnancy and fetal growth measures: a study within the Danish National Birth Cohort

OBJECTIVE

The objective of the study was to examine the association between physical exercise during pregnancy and fetal growth measures.

STUDY DESIGN

Data on 79,692 liveborn singletons from the Danish National Birth Cohort were collected between 1996 and 2002. Mean differences in birthweight, length, ponderal index, head and abdominal circumference, and placental weight and hazard ratios of small- and large-for-gestational-age babies were calculated.

RESULTS

Our data indicated smaller babies in exercising women compared with nonexercisers, but the differences were small, and only a few were statistically significant. Exercising women had a slightly decreased risk of having a child small for gestational age (hazard ratio, 0.87; 95% confidence interval, 0.83-0.92) and large for gestational age (hazard ratio, 0.93; 95% confidence interval, 0.89-0.98).

CONCLUSION

The findings do not indicate sizable effects on fetal growth measures related to exercise apart from a modest decreased risk of small- and large-for-gestational-age infants. These findings do not speak against advising pregnant women to be physically active during pregnancy.

What have birth cohort studies asked about genetic, pre- and perinatal exposures and child and adolescent onset mental health outcomes? A systematic review

Increased understanding of early neurobehavioural development is needed to prevent, identify, and treat childhood psychopathology most effectively at the earliest possible stage. Prospective birth cohorts can elucidate the association of genes, environment, and their interactions with neurobehavioural development. We conducted a systematic review of the birth cohort literature. On the basis of internet searches and 6,248 peer-reviewed references, 105 longitudinal epidemiological studies were identified. Twenty studies met inclusion criteria (prospectively recruited, population-based cohort studies, including at least one assessment before the end of the perinatal period and at least one assessment of behaviour, temperament/personality, neuropsychiatric or psychiatric status before 19 years of age), and their methodologies were reviewed in full. Whilst the birth cohort studies did examine some aspects of behaviour and neurodevelopment, observations in the early months and years were rare. Furthermore, aspects of sampling method, sample size, data collection, design, and breadth and depth of measurement in some studies made research questions about neurodevelopment difficult to answer. Existing birth cohort studies have yielded limited information on how pre- and perinatal factors and early neurodevelopment relate to child psychopathology. Further epidemiological research is required with a specific focus on early neurodevelopment. Studies are needed which include the measures of early childhood psychopathology and involve long-term follow-up.

Environment and obesity in the National Children's Study

We describe the approach taken by the National Children's Study (NCS) to understanding the role of environmental factors in the development of obesity. We review the literature with regard to the two core hypotheses in the NCS that relate to environmental origins of obesity and describe strategies that will be used to test each hypothesis. Although it is clear that obesity in an individual results from an imbalance between energy intake and expenditure, control of the obesity epidemic will require understanding of factors in the modern built environment and chemical exposures that may have the capacity to disrupt the link between energy intake and expenditure. Through its embrace of the life-course approach to epidemiology, the NCS will be able to study the origins of obesity from preconception through late adolescence, including factors ranging from genetic inheritance to individual behaviors to the social, built, and natural environment and chemical exposures. It will have sufficient statistical power to examine interactions among these multiple influences, including gene-environment and gene-obesity interactions. A major secondary benefit will derive from the banking of specimens for future analysis.

Severe obesity in young women and reproductive health: the Danish National Birth Cohort

BACKGROUND

Little is known about reproductive health in severely obese women. In this study, we present associations between different levels of severe obesity and a wide range of health outcomes in the mother and child.

METHODS

From the Danish National Birth Cohort, we obtained self-reported information about prepregnant body mass index (BMI) for 2451 severely obese women and 2450 randomly selected women from the remaining cohort who served as a comparison group. Information about maternal and infant outcomes was also self-reported or came from registers. Logistic regression was used to estimate the association between different levels of severe obesity and reproductive outcomes.

PRINCIPAL FINDINGS

Subfecundity was more frequent in severely obese women, and during pregnancy, they had an excess risk of urinary tract infections, gestational diabetes, preeclampsia and other hypertensive disorders which increased with severity of obesity. They tended to have a higher risk of both pre- and post-term birth, and risk of cesarean and instrumental deliveries increased across obesity categories. After birth, severely obese women more often failed to initiate or sustain breastfeeding. Risk of weight retention 1.5 years after birth was similar to that of other women, but after adjustment for gestational weight gain, the risk was increased, especially in women in the lowest obesity category. In infants, increasing maternal obesity was associated with decreased risk of a low birth weight and increased risk of a high birth weight. Estimates for ponderal index showed the same pattern indicating an increasing risk of neonatal fatness with severity of obesity. Infant obesity measured one year after birth was also increased in children of severely obese mothers.

CONCLUSION

Severe obesity is correlated with a substantial disease burden in reproductive health. Although the causal mechanisms remain elusive, these findings are useful for making predictions and planning health care at the individual level.

Infertility, infertility treatment, and mixed-handedness in children

BACKGROUND

Mixed-handedness, which may reflect atypical brain laterality, has been linked to a number of medical conditions as well as prenatal stress.

AIMS

The aim of the study was to examine whether infertility or infertility treatment was associated with an increased risk of mixed-handedness in children. STUDY DESIGN, SUBJECTS AND OUTCOME MEASURES: We used data from three population-based birth cohorts in Denmark: the Aalborg-Odense Birth Cohort (1984-1987), the Aarhus Birth Cohort (1990-1992) and the Danish National Birth Cohort (1996-2002) (N=7728, 5720 and 29486, respectively). Data on time to pregnancy and infertility treatment were collected during pregnancy. Handedness was reported in a follow-up questionnaire when the children were at least 7years old. Children were categorized as mixed-handed if their mothers reported that they used both hands equally.

RESULTS

Children born after infertility treatment, particularly intrauterine insemination, had a higher risk of being mixed-handed compared to children of fertile couples with a time to pregnancy </=12months (odds ratio 1.41, 95% confidence interval 1.09-1.82). Children of couples with unplanned pregnancies, particularly after an oral contraceptives failure, were also more likely to be mixed-handed. There was no association between a long waiting time to pregnancy and mixed-handedness in children.

CONCLUSIONS

Children born after infertility treatment, particularly intrauterine insemination, and children exposed to oral contraceptives during early gestation may have a higher risk of being mixed-handed.

Polymorphisms in inflammation genes, tobacco smoke and furred pets and wheeze in children

Persistent wheeze in childhood is associated with airway inflammation. The present study investigated relationships between polymorphisms in inflammatory genes, exposure to tobacco smoke and furred pets and risk of recurrent wheeze in children. Within a birth cohort of 101,042 children we identified 1111 eighteen month old cases with recurrent wheeze and 735 wheeze-free controls among 11942 children recruited in the Copenhagen area. Polymorphisms in IL-4R, IL-8, IL-13, SPINK5, and CD14 were genotyped. Interviews at gestational wks 12 and 30, and at age 6 and 18 months included questions on number of episodes with wheeze (18 months), exposure to tobacco smoke and pet-keeping. Recurrent wheeze was defined as at least four episodes of wheeze before the child was 18 months old. There was a statistically significant association between the IL-13 Arg144Gln polymorphism and risk of recurrent wheeze (p = 0.01). Furthermore, there was a statistically significant interaction between this polymorphism and exposure to tobacco smoke during pregnancy, though this was probably a chance finding. There were no other statistically significant effects of the polymorphisms or interactions with exposure to tobacco smoke in relation to the risk of recurrent wheeze. Polymorphisms in IL-8 affected the association between pet-keeping and risk of wheeze. Polymorphisms in inflammation genes might affect the association between environmental exposures and risk of recurrent wheeze in early childhood.

Self-selection and bias in a large prospective pregnancy cohort in Norway

Self-selection in epidemiological studies may introduce selection bias and influence the validity of study results. To evaluate potential bias due to self-selection in a large prospective pregnancy cohort in Norway, the authors studied differences in prevalence estimates and association measures between study participants and all women giving birth in Norway. Women who agreed to participate in the Norwegian Mother and Child Cohort Study (43.5% of invited; n = 73 579) were compared with all women giving birth in Norway (n = 398 849) using data from the population-based Medical Birth Registry of Norway in 2000-2006. Bias in the prevalence of 23 exposure and outcome variables was measured as the ratio of relative frequencies, whereas bias in exposure-outcome associations of eight relationships was measured as the ratio of odds ratios. Statistically significant relative differences in prevalence estimates between the cohort participants and the total population were found for all variables, except for maternal epilepsy, chronic hypertension and pre-eclampsia. There was a strong under-representation of the youngest women (<25 years), those living alone, mothers with more than two previous births and with previous stillbirths (relative deviation 30-45%). In addition, smokers, women with stillbirths and neonatal death were markedly under-represented in the cohort (relative deviation 22-43%), while multivitamin and folic acid supplement users were over-represented (relative deviation 31-43%). Despite this, no statistically relative differences in association measures were found between participants and the total population regarding the eight exposure-outcome associations. Using data from the Medical Birth Registry of Norway, this study suggests that prevalence estimates of exposures and outcomes, but not estimates of exposure-outcome associations are biased due to self-selection in the Norwegian Mother and Child Cohort Study.

An internet-based prospective study of body size and time-to-pregnancy

BACKGROUND

Recent studies have shown that both female and male obesity may delay time-to-pregnancy (TTP). Little is known about central adiposity or weight gain and fecundability in women.

METHODS

We examined the association between anthropometric factors and TTP among 1651 Danish women participating in an internet-based prospective cohort study of pregnancy planners (2007-2008). We categorized body mass index (BMI = kg/m(2)) as underweight (<20), normal weight (20-24), overweight (25-29), obese (30-34) and very obese (> or =35). We used discrete-time Cox regression to estimate fecundability ratios (FRs) and 95% confidence intervals (CI), controlling for potential confounders.

RESULTS

We found longer TTPs for overweight (FR = 0.83, 95% CI = 0.70-1.00), obese (FR = 0.75, 95% CI = 0.58-0.97), and very obese (FR = 0.61, 95% CI = 0.42-0.88) women, compared with normal weight women. After further control for waist circumference, FRs for overweight, obese, and very obese women were 0.72 (95% CI = 0.58-0.90), 0.60 (95% CI = 0.42-0.85) and 0.48 (95% CI = 0.31-0.74), respectively. Underweight was associated with reduced fecundability among nulliparous women (FR = 0.82, 95% CI = 0.63-1.06) and increased fecundability among parous women (FR = 1.61, 95% CI = 1.08-2.39). Male BMI was not materially associated with TTP after control for female BMI. Compared with women who maintained a stable weight since age 17 (-5 to 4 kg), women who gained > or =15 kg had longer TTPs (FR = 0.72, 95% CI = 0.59-0.88) after adjustment for BMI at age 17. Associations of waist circumference and waist-to-hip ratio with TTP depended on adjustment for female BMI: null associations were observed before adjustment for BMI and weakly positive associations were observed after adjustment for BMI.

CONCLUSIONS

Our results confirm previous studies showing reduced fertility in overweight and obese women. The association between underweight and fecundability varied by parity.

[Profile and issues of prospective birth cohort in Japan]

Recently, the effects of environmental risk factors including chemical compounds and parents' lifestyles on the health of the next generations have widely gathered public concerns around the world because of the vulnerability of children to such environmental risk factors. To elucidate the processes and mechanisms of the effects in more detail, the authors started a prospective birth cohort study, namely, the Hokkaido study of Environment and Children's Health in Hokkaido, the northern area of Japan. The study consists of two cohorts: a large-scale cohort throughout Hokkaido and a hospital-based small-scale cohort. The former was established in 2003 in collaboration with forty obstetric hospitals and clinics around Hokkaido to estimate the prevalence of congenital malformations and investigate the association of congenital anomalies with environmental risk factors in pregnant mothers at a background level. The latter was lunched in 2002 at the time of enrollment of pregnant women recruited at an obstetric hospital in Sapporo so as to examine the relationships of environmental substances such as polychlorinated biphenyls (PCBs), dioxins, persistent organic pollutants (POPs), and heavy metals with infants' and children's health outcomes including birth size, neurobehavioral development, thyroid function, and immunologic system. In the study of both cohorts, we attempt to determine the role of gene polymorphism on the occurrence of adverse outcomes in infants and children. Although the prospective cohort study with well-designed epidemiological protocols may provide many scientific lines of evidence, many human and financial resources are required to support the study until its completion and maintain the biobanks as well as data banks. In Japan, it is urgently necessary to establish a system that supports the implementation and management of a cohort study.

The social gradient in birthweight at term: quantification of the mediating role of maternal smoking and body mass index

BACKGROUND

Maternal education is associated with the birthweight of offspring. We sought to quantify the role of maternal body mass index (BMI) and smoking as intermediary variables between maternal education and birthweight at term.

METHODS

We examined the association between maternal education, BMI, smoking and offspring's birthweight among women who delivered at term in the Danish National Birth Cohort (n = 75,085).

RESULTS

Compared with mothers with more than 12 years of education, women with 10-12 years of education had babies that were 12 (4-19) g lighter. Mothers with 9 years of education had babies that were 51 (95% CI; 39-62) g lighter. BMI and smoking affected the association between maternal education and birthweight, albeit in different directions. If all mothers had the BMI of the highest educated mothers, the deficits would be larger: -20 (-22 to -19) and -74 (-80 to -68) g, respectively. If all mothers smoked like the highest educated mothers, mothers with a shorter education would have the heaviest babies: the difference would be 9 (2-16) and 23 (11-36) g, respectively. In combination, smoking and BMI all but explained the educational gradient in birthweight at term.

CONCLUSION

Maternal smoking and BMI are important intermediates of the educational gradient in birthweight at term. As the prevalence of smoking is dropping and the prevalence of obesity is increasing the educational gradient will likely reverse, but it seems unlikely that this will translate into a health advantage for the children of the least educated mothers.