Placenta weight percentile curves for singleton deliveries

OBJECTIVE

To produce population-based, gender- and gestational-age-specific centile curves for placental weight.

DESIGN

Population study.

SETTING

Medical Birth Registry of Norway.

POPULATION

All singleton live births in Norway from 1 January 1999 to 31 December 2002.

METHODS

In a cohort of children born in Norway, placental weights and the ratio of the birthweight to the placental weight were analysed to produce percentile curves.

MAIN OUTCOME MEASURES

Placental weight, birthweight-to-placental weight ratio.

RESULTS

Tables and figures are presented for placental percentiles curves according to gestational age and gender. Also, tables and figures are presented for the ratio of birthweight to placental weight. CONCLUSIONS To our knowledge, this is the first time that population percentile curves have been produced for placental weights and hence for the ratio of birthweight to placental weight. These percentile curves may act as a reference for other populations as well until population-specific curves can be produced.

Changes in parental risk behaviour after an information campaign against sudden infant death syndrome (SIDS) in Norway

AIM

To assess parental risk behaviour before and after a sudden infant death syndrome (SIDS) information campaign with special emphasis on associations with maternal age, education, marital status and birth order.

METHODS

Data from questionnaires sent to all mothers who gave birth in Norway during a period before the campaign were compared with corresponding data obtained after the campaign.

RESULTS

Prevalence of non-supine sleeping position decreased from 33.7% to 13.6% while changes in smoking, non-breastfeeding and co-sleeping were disappointing. Risk factors were particularly prevalent in young mothers, but also in mothers with a minimum period of education, non-cohabitation and at birth order 2+.

CONCLUSIONS

Non-supine sleeping decreased to a level that has never been reported before. In future campaigns, subgroup-specific measures may be needed.

Circadian variations in sudden infant death syndrome: associations with maternal smoking, sleeping position and infections. The Nordic Epidemiological SIDS Study

AIM

To study circadian variation in the sudden infant death syndrome (SIDS) and possible associations with risk factors for SIDS.

METHODS

A questionnaire-based case-control study matched for place of birth, age and gender was conducted in Denmark, Norway and Sweden: The Nordic Epidemiological SIDS Study. The study comprised 244 SIDS victims and 869 control infants between September 1992 and August 1995. The main outcome was hour found dead.

RESULTS

A significant circadian pattern was observed among the 242 SIDS victims with a known hour found dead, with a peak at 08.00-08.59 in the morning (n = 33). Of the SIDS victims, 12% were found dead at 00.00-05.59, 58% at 06.00-11.59, 21% at 12.00-17.59 and 9.0% at 18.00-23.59. When comparing night/morning SIDS and day/evening SIDS (found dead 00.00-11.59 and 12.00-23.59, respectively), the proportion of night/morning SIDS was high among infants of smoking mothers (81% vs 53%, p < 0.001), infants with a reported cold (82% vs 64%, p = 0.007) and infants sleeping side/supine (81% vs 60%, p < 0.001). No associations were observed between hour found dead and other sociodemographic risk factors for SIDS. Risk (odds ratio and 95% confidence interval) of night/morning SIDS and day/evening SIDS was 7.0 (4.5-10.9) and 1.5 (0.8-2.5), respectively, for maternal smoking, 2.2 (1.5-3.1) and 0.6 (0.3-1.3), respectively, if the infant had a reported cold, 3.7 (2.1-6.6) and 3.1 (1.1-8.4), respectively, if the infant was put to sleep in the side position (supine reference), and 11.0 (5.9-20.2) and 21.6 (7.6-60.8), respectively, if the infant was put to sleep in the prone position.

CONCLUSION

The observed higher proportion of night/morning cases in SIDS if the mother smoked, if the infant was reported to have a cold and if the infant was sleeping side/supine may contribute to the understanding of some epidemiological characteristics of SIDS.

Vitamin A and sudden infant death syndrome in Scandinavia 1992-1995

AIM

To assess the effect of vitamin supplementation on the risk of sudden infant death syndrome (SIDS).

METHODS

The analyses are based on data from the Nordic Epidemiological SIDS Study, a case-control study in which parents of SIDS victims in the Scandinavian countries were invited to participate together with parents of four matched controls between 1 September 1992 and 31 August 1995. The odds ratios presented are computed by conditional logistic regression analysis.

RESULTS

The crude odds ratio in Scandinavia for not giving vitamin substitution was 2.8 (95% CI (1.9, 4.3)). This effect was statistically significant in Norway and Sweden, which use A and D vitamin supplementation, but not in Denmark, where only vitamin D supplementation is given. The odds ratios remained significant in Sweden when an adjustment was made for confounding factors (OR 28.4, 95% CI (4.7, 171.3)).

CONCLUSION

We found an association between increased risk of sudden infant death syndrome and infants not being given vitamin supplementation during their first year of life. This was highly significant in Sweden, and the effect is possibly connected with vitamin A deficiency. This effect persisted when an adjustment was made for potential confounders, includingsocioeconomic factors.

Pre-eclampsia in pregnancy and subsequent risk for breast cancer

Women who experience pre-eclampsia or hypertension during pregnancy may have a reduced risk for breast cancer later in life. The evidence is based on case-control studies, and here we report the results of a cohort study exploring the link between pre-eclampsia and gestational hypertension diagnosed in the first pregnancy and subsequent risk for breast cancer. We combined information from the Medical Birth Registry and the Cancer Registry in Norway, which are both nation-wide. Between 1967, when the birth registry was established, and 1998, 694 657 women were recorded with a first birth, and classified according to whether pre-eclampsia and/or hypertension was diagnosed in the first pregnancy. Linkage to the Norwegian Cancer Registry identified 5474 new cases of breast cancer diagnosed subsequently to their first delivery. Compared to other parous women, women with pre-eclampsia and/or hypertension diagnosed in their first pregnancy had 19% lower risk (95% confidence interval, 9 to 29%) for breast cancer, after adjustment for attained age, calendar period of diagnosis, age at first birth, and parity. This result was similar for term and preterm deliveries, across the range of offspring birth weight, and for pre- and postmenopausal women. These results suggest that the pathophysiology surrounding pre-eclampsia and gestational hypertension plays an important role in breast cancer etiology. A better understanding of the underlying processes could provide an insight into the pathogenesis of breast cancer.

Pregnancy related protection against breast cancer depends on length of gestation

In a prospective study of 694 657 parous women in Norway, 5474 developed breast cancer after their first birth. If the first pregnancy lasted less than 32 weeks, the risk was 22% (95% confidence interval, -3% to 53%) less than after a pregnancy of 40 weeks or more, with a significant declining trend in risk (P for trend=0.02).

Breast feeding and the sudden infant death syndrome in Scandinavia, 1992-95

AIMS

To assess the effects of breast feeding habits on sudden infant death syndrome (SIDS).

METHODS

The analyses are based on data from the Nordic Epidemiological SIDS Study, a case-control study in which parents of SIDS victims in the Scandinavian countries between 1 September 1992 and 31 August 1995 were invited to participate, each with parents of four matched controls. The odds ratios presented were computed by conditional logistic regression analysis.

RESULTS

After adjustment for smoking during pregnancy, paternal employment, sleeping position, and age of the infant, the adjusted odds ratio (95% CI) was 5.1 (2.3 to 11.2) if the infant was exclusively breast fed for less than four weeks, 3.7 (1.6 to 8.4) for 4-7 weeks, 1.6 (0.7 to 3.6) for 8-11 weeks, and 2.8 (1.2 to 6.8) for 12-15 weeks, with exclusive breast feeding over 16 weeks as the reference. Mixed feeding in the first week post partum did not increase the risk.

CONCLUSIONS

The study is supportive of a weak relation between breast feeding and SIDS reduction.

Paternal contribution to birth weight

STUDY OBJECTIVE

Understanding causes of variation in birth weight has been limited by lack of sufficient sets of data that include paternal birth weight. The objective was to estimate risks of low birth weight dependent on parental birth weights and to estimate father-mother-offspring correlations for birth weight to explain the variability in birth weight in terms of effects of genes and environmental factors.

DESIGN

A family design, using trios of father-mother-firstborn child.

SETTING

The complete birth population in Norway 1967-98.

PARTICIPANTS

67 795 families.

MAIN RESULTS

The birth weight correlations were 0.226 for mother-child and 0.126 for father-child. The spousal correlation was low, 0.020. The relative risk of low birth weight in the first born child was 8.2 if both parents were low birth weight themselves, with both parents being above 4 kg as the reference. The estimate of heritability is about 0.25 for birth weight, under the assumption that cultural transmission on the paternal side has no effect on offspring prenatal growth.

CONCLUSIONS

Paternal birth weight is a significant and independent predictor of low birth weight in offspring. The estimate of the heritability of birth weight in this study is lower than previously estimated from data within one generation in the Norwegian population.

Premorbid body weight and its relations to primary tumour diameter in breast cancer patients; its dependence on estrogen and progesteron receptor status

Hormonal mechanisms have been offered as an explanation for the higher frequency of large tumours, lymph node metastases and poorer prognosis in obese breast cancer patients than in lean ones. If hormonal mechanisms are important for these relations, they should probably act more strongly in patients with hormonal receptor positive tumours than in those with negative ones. We have examined if the relations between premorbid body weight or Quetelet's index (weight/height2) and tumour diameter are modified by estrogen receptor alpha (ER) and progesteron receptor (PgR) status. The analyses were based on 1,241 women with unilateral disease treated with modified radical mastectomy living in the geografic area of Haukeland Hospital. Their body weight and height have been measured as a mean 12.5 years before presentation of the disease. Body weight and Quetelet's index have been adjusted for age. The relations were studied using linear regression analyses adjusting the effect of body weight with height and mean nuclear area of the tumour cells and adjusting the effect of Quetelet's index for mean nuclear area. The main findings showed that patients with high body weight or Quetelet's index presented more often with PgR positive tumours than lean ones. Quetelet's index was also positively related to ER. These relations were present in patients older than 50 years of age (older). Patients with large tumours (>2.0 cm) had significantly higher body weight and Quetelet's index than those with small ones. These differences were significantly present in older patients and in patients with PgR negative and ER negative-PgR negative tumours. Linear regression analyses confirmed that tumour diameter increases with body weight and Quetelet's index. These relations were present in both lymph node groups and in older patients. Stratification according to hormonal receptor status showed these relations to be significant in patients with ER negative, with PgR negative and those with ER negative-PgR negative tumours only. Taking age and hormonal receptor status into consideration simultaneously, both body weight and Quetelet's index were significantly related to tumour diameter in older patients with hormone receptor negative tumours. In conclusion body size was positively related to hormone receptor status and to diameter of the primary tumour. The relation to tumour diameter was present in older patients with hormone receptor negative tumours. Although hormonal mechanisms able to act on the tumour can not be excluded, mechanisms acting independent of hormonal receptors must be considered. Different mechanisms related to body fat cytokines are discussed.

Survival and reproduction among males with birth defects and risk of recurrence in their children

CONTEXT

Few systematic data exist on survival and reproduction among males with birth defects and their contribution to occurrence of birth defects in the next generation.

OBJECTIVE

To estimate survival of males with registered birth defects, their subsequent reproduction rates, and their risk of transmitting birth defects to their offspring.

DESIGN AND SETTING

Population-based cohort study of data from the Medical Birth Registry of Norway.

SUBJECTS

A total of 486 207 males born in Norway between 1967 and 1982, 12 292 of whom had a recorded birth defect.

MAIN OUTCOME MEASURES

Survival rates through 1992, reproduction rates through 1998, and risk of birth defects among offspring of males with vs without birth defects.

RESULTS

Survival through 1992 was lower among males with birth defects (84% vs 97%). Compared with males without birth defects, affected males were 28% less likely to have had a child. Among offspring of affected males, 5.1% had a registered birth defect compared with 2.1% of offspring of males without birth defects (relative risk [RR], 2.4; 95% confidence interval [CI], 1.9-3.0). Offspring of affected fathers had an increased risk of the same defect as their fathers (RR, 6.5; 95% CI, 4.0-10.4) and an increased risk of dissimilar defects (RR, 1.8; 95% CI, 1.3-2.5).

CONCLUSIONS

Compared with unaffected males, males with birth defects have higher mortality and survivors are less likely to have a child. Fathers with birth defects are significantly more likely than unaffected fathers to have an affected child.

Changes in the epidemiology of sudden infant death syndrome in Sweden 1973-1996

BACKGROUND

From the early 1970s to the early 1990s, there was a significant rise in the incidence of sudden infant death syndrome (SIDS) in Scandinavia. Following the risk reducing campaign, the incidence has fallen to about the same level as in 1973.

AIMS

To identify the changes that have occurred in the epidemiology of SIDS.

METHODS

We compared the Swedish part of the Nordic Epidemiological SIDS Study (NESS), covering the years 1992-1995, with two earlier, descriptive studies during this period. To assess the changing effects of risk factors, we analysed data from the Medical Birth Registry of Sweden, covering the years 1973-1996.

RESULTS

There was a predominance of deaths during weekends in the 1970s and 1990s. The seasonal variation was most notable in the 1980s. The proportion of young mothers decreased from 14% to 5%. Cohabitation (living with the biological father) was as frequent in the 1990s as in the 1970s. The prevalence of high parity, admissions to neonatal wards, low birth weight, prematurity, and multiple pregnancies were all increased in the 1990s compared to the 1970s. No significant change in the prevalence of previous apparent life threatening events was found. Deaths occurring in cars diminished from 10% to below 2%. In the data from the Medical Birth Registry of Sweden, there were significantly increased odds ratios after the risk reducing campaign of the risk factors smoking during early pregnancy and preterm birth. We could find no increased effects of maternal age, parity, or being small for gestational age over time. The rate of deaths at weekends remained increased; the median age at death fell from 90 to 60 days. Seasonal variation was less notable in the periods of low incidence.

New standards for birth weight by gestational age using family data

OBJECTIVE

This study was undertaken to provide new standards for birth weight according to gestational age through the addition of family data on maternal birth weight and birth weights of previous siblings.

STUDY DESIGN

The analyses were based on 1.7 million births in Norway from 1967 through 1998. These population data were arranged into sibships and mother-offspring units through unique personal numbers. We categorized first births by sex and maternal birth weight and second births by sex and birth weight of the older sibling.

RESULTS

Standards for birth weight per gestational age percentiles differed by >1100 g when the birth weight of an older sibling was considered and by almost 700 g when maternal birth weight was considered. The value of these new standards for birth weight according to gestational age was demonstrated through variation in perinatal mortality.

CONCLUSION

Maternal birth weight and birth weights of previous siblings allow improved predictions of birth weight according to gestational age and should be used for classification of small-for-gestational-age births.

Analyzing sibship correlations in birth weight using large sibships from Norway

Data from the Medical Birth Registry of Norway were used to estimate sibship correlations in large sibships (each with > or = 5 infants among singleton live births surviving the first year of life), while adjusting for covariates such as infant gender, gestational age, maternal age, parity, and time since last pregnancy. This sample of 12,356 full sibs in 2,462 sibships born in Norway between 1968 and 1989 was selected to maximize the information on parity, and a robust approach to estimating both regression coefficients and the sibship correlation using generalized estimating equations (GEE) was employed. In concordance with previous studies, these data showed a high overall correlation in birth weight among full sibs (0.48 +/- 0.01), but this sibship correlation was influenced by parity. In particular, the correlation between the firstborn infant and a subsequent infant was slightly lower than between two subsequent sibs (0.44 +/- 0.01 vs. 0.50 +/- 0.01, respectively). The effect of time between pregnancies was statistically significant, but its predicted impact was modest over the period in which most of these large families were completed. While these data cannot discriminate whether factors influencing birth weight are maternal or fetal in nature, this analysis does illustrate how robust statistical models can be used to estimate sibship correlations while adjusting for covariates in family studies.

Maternal smoking and birthweight: effect modification of period, maternal age and paternal smoking

OBJECTIVE

To study the effect on birthweight of maternal smoking, and its modification by study period, maternal age and paternal smoking.

DESIGN

A retrospective questionnaire based national survey comprising a random sample (n=34,799) of all mothers giving birth in Norway 1970-91. Variables studied were parental smoking during pregnancy, birthweight, maternal age and infant's year of birth.

RESULTS

The overall difference in mean birthweight between non-smoking and smoking mothers was 197 g. The difference in birthweight between non-smoking and smoking mothers increased with maternal age from 182 g (<20 years of age) to 232 g (35+ years of age). There was no significant effect of paternal smoking on birthweight when the mother was a non-smoker. When the mother was a smoker and the father was a non-smoker, the birthweight, adjusted for maternal age, was reduced by 153 g (p<0.005). However, when both parents smoked, the birthweight, adjusted for maternal age, was reduced by 201 g (p<0.0005). Even though the prevalence of paternal smoking decreased by 38% during the study period, there was no significant increase in overall mean birthweight. IMPLICATION AND RELEVANCE OF RESULTS: The negative effect of maternal smoking on birthweight appears to increase with maternal age. For a non-smoking pregnant woman to live with a smoking partner has little, if any, effect on birthweight. The negative effect of paternal smoking was only observed when the mother was smoking and might reflect two possible mechanisms: (1) that a smoking mother has a greater cigarette consumption when the partner also smokes, and (2) that a smoking mother is less concerned about passive smoking than a non-smoking mother.

Birthweight by gestational age in Norway

OBJECTIVE

To describe birthweight by gestational age in Norway for the period 1967-1998, evaluate secular trends and provide new standards for small for gestational age for 16 to 44 weeks of gestation.

SUBJECTS AND METHODS

The analyses were based on more than 1.8 million singleton births, covering all births in Norway for a 32 year period. Percentiles for birthweight by gestational age were estimated using smoothed means and standard deviations. In the preterm weeks, means and standard deviations were carefully screened for birthweight-gestational age consistency, adapting a method of Wilcox and Russell. Differences in birthweight by gestational age for stillbirths and livebirths in extremely preterm weeks (16-28) are presented, and the effects of cesarean section are evaluated. We observed a clear increase in birthweight by gestational age for all term weeks, but a decrease for most of the preterm weeks over the same period. This decrease was related to the increase in deliveries by cesarean section.

CONCLUSIONS

Percentiles for birthweight by gestational age are presented for clinical use, based on a current period 1987-98, covering 20-44 completed gestational weeks. In the final standards we excluded stillbirths, infants born with malformations and cesarean sections. Birthweights in the Scandinavian populations are high and standards from other populations may not be representative, especially for the term weeks. Also, the secular changes demonstrated in this study indicate that old birthweight by gestational age standards need revision, especially due to changes in obstetrical routines influencing preterm data.

Birthweight percentiles by gestational age in multiple births. A population-based study of Norwegian twins and triplets

OBJECTIVE

To assess secular trends for birthweight by gestational age in twins in Norway and to develop current national birthweight standards by gestational age for twin and triplet births using population-based data.

MATERIAL AND METHODS

The analysis of secular trends for birthweight and gestational age in twins was based on 32,379 twin livebirths (1967-95). Taking into account the observed secular trends in birthweight for 35-40 weeks of gestation, data on twins born during 1987-95 only were included in the calculation of birthweight percentiles for 35-40 weeks, while for lower and upper weeks, data on twins born during 1967-95 were used. The construction of birthweight-for-gestation curves for triplets was based on the data on 690 triplets.

RESULTS

Whereas the overall mean birthweight and gestational age decreased in 1987-95 compared with the previous years, the mean birthweights by gestational age for the 35-40 weeks of gestation was significantly higher in 1987-95. Male twins weighed more than female twins throughout the gestation with consistent and significant differences from 27 to 42 weeks of gestation. Smoothed curves for birthweight-by-gestational-age percentiles of male and female twins are plotted. The birthweight-by-gestational-age curves of triplets were almost identical with twin curves before 30 weeks of gestation, starting to diverge from them progressively thereafter. The intrauterine growth of twin births also starts to differ markedly from singletons at approximately 30 weeks of gestation.

CONCLUSION

This study shows that plurality-specific birthweight-by-gestation standards should be used for assessment of fetal growth in multiple births rather than singleton standards.

Infants' length at birth: an independent effect on perinatal mortality

AIM

To investigate whether variations in birth length (crown-heel-length) were associated with perinatal mortality rate independent of birth weight.

MATERIAL

The study population was singleton live- and stillbirths from 16 weeks of gestation compiled in the Medical Birth Registry of Norway from 1967 to 1997, totaling 1,705,652 births.

METHOD

The total population was analyzed using z-scores for length at birth, birth weight and gestational age. Variation in perinatal mortality by length at birth was studied within birth weight strata (250 g) by logistic regression.

RESULTS

Perinatal mortality varied more by birth length than by birth weight or gestational age, especially for values above the population means. Within birth weight strata, the association between perinatal mortality and length was similar in all 250 g birth weight categories above 1,500 grams: mortality was lowest at birth lengths 0-2 cm below average, with mortality rates increasing exponentially in either direction.

CONCLUSION

Within all birth weight strata, and adjusted for gestational age, long infants had the higher risk of perinatal death, suggesting that length at birth may be a valuable predictor when assessing the risk of perinatal mortality.

Recurrence of gestational age in sibships: implications for perinatal mortality

The authors studied the extent to which preterm birth and perinatal mortality are dependent on the gestational ages of previous births within sibships. The study was based on data collected by the Medical Birth Registry of Norway from 1967 to 1995. Newborns were linked to their mothers through Norway's unique personal identification number, yielding 429,554 pairs of mothers and first and second singleton newborns with gestational ages of 22-46 weeks, based on menstrual dates. Siblings' gestational ages were significantly correlated (r = 0.26). The risk of having a preterm second birth was nearly 10 times higher among mothers whose firstborn child had been delivered before 32 weeks' gestation than among mothers whose first child had been born at 40 weeks. However, perinatal mortality in preterm second births was significantly higher among mothers whose first infant had been born at term, compared with mothers whose firstborn child was delivered at 32-37 weeks. Since perinatal mortality among preterm infants is dependent on the gestational age in the mother's previous birth, a common threshold of 37 weeks' gestation for defining preterm birth as a risk factor for perinatal death may not be appropriate for all births to all mothers.

Caffeine and alcohol as risk factors for sudden infant death syndrome. Nordic Epidemiological SIDS Study

OBJECTIVE

To assess whether alcohol and caffeine are independent risk factors for sudden infant death syndrome (SIDS).

MATERIALS AND METHODS

Analyses based on data from the Nordic epidemiological SIDS study, a case control study in which all parents of SIDS victims in the Nordic countries from 1 September 1992 to 31 August 1995 were invited to participate with parents of four controls, matched for sex and age at death. Odds ratios (ORs) were calculated by conditional logistic regression analysis.

RESULTS

The crude ORs for caffeine consumption > 800 mg/24 hours both during and after pregnancy were significantly raised: 3.9 (95% confidence interval (CI), 1.9 to 8.1) and 3.1 (95% CI, 1.5 to 6.3), respectively. However, after adjustment for maternal smoking in 1st trimester, maternal age, education and parity, no significant effect of caffeine during or after pregnancy remained. For maternal or paternal alcohol use, no significant risk increase was found after adjusting for social variables, except for heavy postnatal intake of alcohol by the mother, where the risk was significantly increased.

CONCLUSIONS

Caffeine during or after pregnancy was not found to be an independent risk factor for SIDS after adjustment for maternal age, education, parity, and smoking during pregnancy. Heavy postnatal but not prenatal intake of alcohol by the mother increased the risk.

Impact of multiple births and elective deliveries on the trends in low birth weight in Norway, 1967-1995

To describe trends in low birth weight (less than 2,500 g), the authors analyzed 1.7 million live births and stillbirths registered between 1967 and 1995 in the Medical Birth Registry of Norway. The proportion of low birth weight infants declined from 5.3% in 1967 to 4.5% in 1979 and was followed by a steady increase that reached 5.3% in 1995. Similar trends were observed in the proportion of preterm births. Mean birth weight increased from 3,456 g in 1967 to 3,518 g in 1995. From 1979 to 1987, the increase in the prevalence of low birth weight was related to single births, and after 1987 it was related to multiple births, which increased from 2.3% of all births in 1987 to 3.1% in 1995. The proportion of low birth weight in births occurring after 37 weeks of gestation declined continuously, resulting in low birth weight births' to an increasing extent being made up of births occurring before 37 weeks of gestation. In an ecologic analysis based on county of maternal residence, the increase in low birth weight among single births was accounted for by an increase in deliveries with induction of labor or cesarean section. The authors conclude that the overall proportion of low birth weight births is not a good indicator of health in a population with extensive use of obstetric procedures that affect gestational age or assisted fertilization, which increases the number of multiple births.

Interactions of infectious symptoms and modifiable risk factors in sudden infant death syndrome. The Nordic Epidemiological SIDS study

The aim of the study was to investigate the effect of infection on sudden infant death syndrome (SIDS) and to analyse whether modifiable risk factors of SIDS, prone sleeping, covered head and smoking act as effect modifiers. In a consecutive multicentre case-control study of SIDS in Denmark, Norway and Sweden, questionnaires on potential risk factors for SIDS were completed by parents of SIDS victims, and for at least two controls matched for gender, age and place of birth. All SIDS cases were verified by an autopsy. The study comprised 244 SIDS cases and 869 controls, analysed by conditional logistic regression. Significantly more cases than controls presenting symptoms of infectious diseases during the last week and/or last day were treated with antibiotics and had been seen by a physician. The finding is consistent with the hypothesis of an infectious mechanism in SIDS induced by local microorganism growth and toxin or cytokine production, and also adds further support to a possible association between infection and SIDS by loss of protective mechanisms, such as arousal. The risk of SIDS among infants with the combined presence of infectious symptoms and either of the other modifiable risk factors, prone sleeping, head covered or parental smoking, was far greater than the sum of each individual factor. These risk factors thus modify the dangerousness of infection in infancy.

A population-based study of survival and childbearing among female subjects with birth defects and the risk of recurrence in their children

BACKGROUND AND METHODS

Persons with birth defects are at high risk for death during the perinatal period and infancy. Less is known about the later survival or reproduction of such persons. We studied a cohort that comprised 8192 women and adolescent girls with registered birth defects and 451,241 women and adolescent girls with no birth defects, all of whom were born in Norway from 1967 through 1982. The rate of survival was determined through 1992, and the rate of childbearing was determined through October 1997. We also estimated the risk of birth defects in the children of these subjects.

RESULTS

Among the subjects with birth defects, 80 percent survived to 15 years of age, as compared with 98 percent of those with no birth defects. Among the surviving subjects, 53 percent of those with birth defects gave birth to at least one infant by the age of 30 years, as compared with 67 percent of those with no birth defects. The subjects with birth defects were one third less likely to give birth by the age of 30 than those with no birth defects. The children of the subjects with birth defects had a significantly higher risk of birth defects than the children of those with no birth defects (relative risk, 1.6; 95 percent confidence interval, 1.3 to 2.1). This increased risk was confined entirely to the specific defect carried by the mother, with the relative risk of recurrence varying from 5.5 to 82 according to the defect. In contrast, there was no increase in the risk of having an infant with a different type of defect.

CONCLUSIONS

Women and girls with birth defects have decreased survival as compared with those with no birth defects, especially in the first years of life, and are less likely to have children. In addition, they have an increased risk of having children with the same defect.

Errors in gestational age: evidence of bleeding early in pregnancy

OBJECTIVES

This study explored the extent of errors in gestational age as ascertained by last menstrual period.

METHODS

More than 1.5 million birth records (covering the years 1967-1994) from the population-based Medical Birth Registry of Norway were used to study variation in gestational age within strata of birthweight.

RESULTS

Within 100-g strata of birthweight, it was found that the observed gestational age distribution could be divided into 3 distinct underlying distributions separated by approximately 4 weeks. This pattern was present through all birthweight strata, from 200 g up to 4700 g. In addition, the apparent misclassification causing a gestational age 4 weeks too short was much more common among low-birthweight births than among heavier births.

CONCLUSIONS

The separation of the gestational age distributions by intervals of close to 4 weeks suggests that errors in gestational age measurements are caused by factors related to menstrual bleeding. Furthermore, there is evidence for a strong relation between bleeding at the time of the next menstrual period after conception and low birthweight. This conclusion should be approached with caution because of the retrospective nature of the data.