Intergenerational influences affecting birth outcome. II. Preterm delivery and gestational age in the children of the 1958 British birth cohort

The 1958 British cohort study has data to investigate intergenerational effects on preterm delivery and on gestational age in non-preterm births, allowing for many confounders that may differ in the more pathological preterm babies. Previous results for all gestational ages have been inconsistent. The strongest and only likely independent intergenerational effect on non-preterm gestational age found is parental gestational age (adjusted regression coefficient = 0.067 weeks per week in mothers and 0.045 in fathers). The preterm analysis has low power; however, reported history of hypertension in mothers (any), in fathers and in the maternal grandmother (measured in the 1958 pregnancy) all significantly and independently increased the risk of preterm birth [OR = 1.7, 2.0, 1.5 respectively]. The absolute risk was particularly high in hypertensive mothers who had been preterm themselves (21%). Other possible intergenerational influences of height, weight, fetal growth and gestation were not significant enough and/or consistent enough between parents to speculate whether they are truly intergenerational or confounded by other factors acting during the pregnancy. Excepting mother's weight for height, no genetic or environmental influence studied affects both gestational age and fetal growth in term births. However, many maternal factors that reduce either fetal growth or gestation in term births are associated with increased risk of preterm birth.     

Birth weight and adult cardiovascular risk factors using multiple birth status as an instrumental variable in the 1958 British Birth Cohort

BackgroundBirth weight is classified as a risk factor for cardiovascular disease by the World Health Organization, but appropriate preventive interventions remain unclear because the observations have not been confirmed in experiments and appear to be contextually specific.MethodsUsing 9452 participants of the 1958 British Birth Cohort at age 42 years in 2000 (58% follow-up), we examined the credibility of multiple birth status as an instrumental variable (IV) for birth weight and, if appropriate, use it to obtain less confounded estimates of the associations of birth weight with cardiovascular disease risk factors including self-reported height, body mass index and hypertension than conventional regression in 2014.ResultsMultiple birth (203 twins and 6 triplets) was associated with older maternal age, but not with paternal occupation or maternal smoking. Multiple births had lower birth weight-for-gestational age z-score. Multiple birth status was not directly associated with height, BMI or hypertension. Using IV estimates birth weight-for-gestational age z-score was not clearly associated with height (0.99 cm, 95% confidence interval (CI) − 0.27, 2.25), body mass index (BMI) (0.42 kg/m², 95% CI − 0.17, 1.01) or hypertension (risk ratio 0.82, 95% CI 0.54, 1.23) adjusted for maternal age, with a first-stage F statistic of 145.3 from IV analysis.ConclusionsMultiple birth status is a credible IV for obtaining a less confounded estimate of the association of birth weight with height, BMI and blood pressure. Such analysis suggests that birth weight may be spuriously related to height, BMI and blood pressure, and thus not an effective target for intervention.     

Birthweight percentiles by gestational age and gender for children in the North of Mexico

The objective of this study was to determine the 10th, 50th and 90th percentiles of birthweight, by gestational age and sex, for newborns covered by the Mexican Institute of Social Security (IMSS) in the State of Chihuahua. To generate the database, we used IMSS hospitals' records in the State of Chihuahua, covering the period between 1 January 2000 and 31 December 2004. We included singleton live births only, and excluded babies with congenital malformations. The birthweights of 88,368 children born at 21-44 weeks of gestation comprised our data. From these data, we calculated the 10th, 50th and 90th percentiles for each sex, at 32-44 weeks of gestation. The observed cutoffs for the 10th percentile in our population were 40-250 g higher than those reported in other references with Mexican populations. These results constitute an updated birthweight reference that will allow the identification of newborns in the North region of the country with low birthweight-for-gestational age. Such information can be a useful instrument for preventing or diminishing associated risks.     

Life-Course Partnership Status and Biomarkers in Midlife: Evidence From the 1958 British Birth Cohort

Objectives. We examined the association between trajectories of partnership status over the life course and objectively measured health indicators in midlife.Methods. We used data from 4 waves (1981, 1991, 2000, and 2002–2004) of the British National Child Development Study (NCDS), a prospective cohort study that includes all people born in Britain during 1 week in March 1958 (n = 18 558).Results. After controlling for selection attributable to early-life and early-adulthood characteristics, we found that life-course trajectories of partnership status were associated with hemostatic and inflammatory markers, the prevalence of metabolic syndrome and respiratory function in midlife. Never marrying or cohabiting was negatively associated with health in midlife for both genders, but the effect was more pronounced in men. Women who had married in their late 20s or early 30s and remained married had the best health in midlife. Men and women in cohabiting unions had midlife health outcomes similar to those in formal marriages.Conclusions. Partnership status over the life course has a cumulative effect on a wide range of objectively measured health indicators in midlife.Numerous studies have found that married people have better health and lower mortality than unmarried people, and these findings have been replicated in different countries and time periods.1–18 A reduction in health inequalities related to marital status therefore has the potential to shift the distribution of risk and improve population health.19 However, to do so, further understanding of the mechanisms that link marital status and health is needed, including further consideration of health-related selection into various marital statuses, the operation of health-protective effects of marriage, and the accumulation of benefits and risks of marital status trajectories over the life course.With a few exceptions,20 studies of marital status and health have considered only current marital status or transitions over relatively short periods,15 and only a few studies have considered the association between nonmarital cohabitation and health,12,21 a topic of increasing importance given that cohabitation is becoming more common in the United Kingdom.22 Furthermore, of those studies that have used measures of health, rather than mortality, as an outcome,23 most have used self-reported measures. In the few studies in which objective health indicators were used, sample sizes were relatively small.24,25In this study, we used data from a population-based birth cohort to summarize longitudinal patterns of partnership status that distinguish marital status and nonmarital cohabitation. We used a model-based approach that allowed us to capture stability as well as transitions in partnership status over a 21-year period (ages 23–44 years) and used this to investigate the effects that 21-year trajectories of partnership status have on a wide range of biomarkers in midlife. Our objective was to investigate the association of different trajectories of partnership status over the life course and objectively measured health indicators in midlife.     

Birthweight distributions by gestational age: comparison of LMP-based and ultrasound-based estimates of gestational age using data from the Swedish Birth Registry

Studies based on data from the US have reported that the birthweight distribution at gestational age 28–31 weeks is bimodal with a second peak occurring at approximately 3300 g, suggesting that there is misclassification of term infants. In these studies, gestational ages were estimated from the date of the last menstrual period (LMP), and it has been suggested that ultrasound-based estimates of gestational age would eliminate the problem with bimodal birthweight distributions. Swedish data include both measures, thus offering an opportunity for comparison. All singleton births in Sweden from 1993 to 2002 with information on birthweight were included in the study ( n  = 917 901). Both LMP- and ultrasound-based estimates of gestational age were available for 75.1% of the births. Two possible sources of misclassification were considered: measurement error, assuming that ultrasound-based estimates are better, and data entry errors. An algorithm for assessment of data entry errors was developed; 67.4% of the births were left for the analyses of data 'cleaned' from data entry errors.
Based on the entire study population, the LMP-based birthweight curves for lower-gestational-age preterm births were bimodal, with a second peak around 3500 g. The bimodal distribution was greatly attenuated when using ultrasound-based gestational age categories, but did not disappear. After cleaning the data, the LMP-based birthweight distributions for infants at gestational ages <32 weeks were no longer bimodal, and were very similar to the ultrasound-based curves. In conclusion, data entry errors are more likely to cause the bimodality in the birthweight distribution among preterm infants than measurement errors in the LMP-based gestational age estimate.     

Childhood Cognition and Risk Factors for Cardiovascular Disease in Midadulthood: The 1958 British Birth Cohort Study

Objectives. We sought to establish whether associations between childhood cognition and risk factors for cardiovascular disease in adulthood are explained by common causes, or adult social position or health behavior.Methods. We analyzed associations between cognition at age 11 and cardiovascular disease risk factors at age 45 in the 1958 British birth cohort (n = 9377), with and without adjustment for covariates.Results. General ability was inversely associated with systolic and diastolic blood pressure, glycosylated hemoglobin, triglycerides (in women), body mass index, and waist circumference. Systolic blood pressure decreased by 0.47 mm Hg (95% confidence interval [CI] = −0.90, –0.05) for a 1-standard-deviation increase in ability. Separate adjustment for social class at birth, education level by adulthood, adult social class, and health behaviors reduced the associations respectively by 14% to 34%, 36% to 50%, 14% to 36%, and 24% to 73%. Full adjustment reduced associations between ability and risk factors at age 45 years by 43% to 92%, abolishing all associations.Conclusions. Increments across the distribution of childhood cognition are associated with improvements in cardiovascular risk profile in midlife, with associations primarily mediated through adult health behavior and social destinations.Individuals with a higher IQ in the first 2 decades of life have been found to have lower death rates from all causes in mid- to late adulthood, as reported in a systematic review of 9 prospective studies.¹ Associations between childhood cognition and mortality were not caused by confounding, reverse causality, or selection bias; hence, work directed at understanding the association is needed.Knowledge of the specific diseases associated with childhood cognition may yield clues about the likely explanations. Recent studies have investigated relationships between childhood cognition and intermediate risk factors for later cardiovascular disease. For example, higher childhood cognition has been associated with lower systolic and diastolic blood pressure in the Midspan cohorts of Scottish adults,² although another Scottish study found no association for self-reported hypertension.³ Prevalence of self-reported diabetes was reduced in adults with higher childhood cognition in a US but not a Scottish population.^3,4 Associations between higher childhood cognition and lower levels of adiposity have been observed, with some studies investigating mediation of factors such as adult education.^5–8Explanations have been proposed for associations between childhood cognition and adult mortality or morbidity: first, there may be common causes of cognition and adult disease, including childhood socioeconomic position; second, childhood cognition predicts later educational and occupational attainment, which in turn influences exposure to hazardous environments that affect risk of adult disease; and third, childhood cognition affects the extent to which individuals acquire or act on knowledge about protective or health-damaging behaviors linked to disease.⁹ Few studies have examined explanations in detail, largely because of limited data availability on relevant factors.We, therefore, examined associations between childhood cognition and several risk factors for cardiovascular disease (CVD) in a large population-based sample of men and women. Our primary aim was to establish whether childhood cognition is associated with blood pressure, lipids, glycosylated hemoglobin (HbA_1c), and total or central adiposity in midlife. We sought to determine whether associations between childhood cognition and risk factors for CVD are explained by (1) early life factors, such as birthweight and social origins, which could act as common causes of both cognition and CVD risk; (2) adult social position; or (3) health behaviors. We studied a prospective birth cohort, followed to age 45 years, in which we have previously shown lower birthweight and less-advantaged social origins to be associated with poorer childhood cognition.¹⁰ In turn, poorer cognition was found in this cohort to increase the risk of behaviors, such as abstaining from alcohol and binge drinking¹¹ and smoking,¹² linked to adverse health outcomes.     

Birth weight and gestational age among children of Danish pharmacy assistants.

STUDY OBJECTIVE--The aim was to investigate whether specific types of work or exposures among pharmacy assistants were correlated with changes in birth weight or gestational age. DESIGN--The study was a nationwide historical cohort with open entry of all female members of the union of pharmacy assistants during 1979 to 1984. PARTICIPANTS--The cohort comprised 4939 pharmacy assistants under 40 years of age. The questionnaire was answered by 93%. MEASUREMENTS AND MAIN RESULTS--Information on birth weight and gestational age was obtained by linkage to the national birth register. Exposure information on type of work and exposure to pharmaceutical and chemical products during pregnancy was self reported. Only small and non-significant differences in birth weight and gestational age were found between the compared groups. Mean birth weight was 50 g above the national mean and gestational age was longer (p less than 0.0005). CONCLUSIONS--Compared to the Danish population, children of pharmacy assistants are at low risk for experiencing low birth weight and preterm birth. Social status and less smoking during pregnancy among pharmacy assistants might explain the slightly greater mean birthweight and gestational age in this sector of the population.     

Work in pregnancy and birth weight for gestational age.

In a recent report on prematurity and work in pregnancy based on the Montreal survey noteworthy increases in both preterm births (less than 37 weeks) and infants of low birth weight (less than or equal to 2500 g) were found in women in certain specific occupations or whose work entailed heavy lifting, shift work, long hours, or great fatigue. Because of the large overlap between preterm births and low birth weight, the latter was further analysed with allowance for gestational age in order better to separate factors retarding fetal growth from those shortening gestation. The association of low birth weight with specific occupations, long working hours, and fatigue largely disappeared, suggesting that the effect of these factors was to shorten gestation. By contrast, the association with lifting heavy weights and with shift work persisted, suggesting that these factors retarded fetal growth as well as increasing the risk of preterm birth.     

Drinking histories of self-identified lifetime abstainers and occasional drinkers: findings from the 1958 British Birth Cohort Study

AIMS: To investigate the validity of retrospective items used to distinguish people who have rarely or never consumed alcohol. METHODS: The 1958 British Birth Cohort Study has followed 9377 individuals until age 45. Previous drinking (at 16, 23, 33 and 42 years) was investigated for two groups of 45-year-old non-drinkers, those reporting never having consumed alcohol ('never drinkers', n = 143, 1.5%), and having only consumed very infrequently ('occasional-only drinkers', n = 1149, 12.3%). RESULTS: 67% of never drinkers previously reported drinking, 25% were past weekly/daily drinkers; 56% of occasional-only drinkers reported weekly/daily consumption. The validity of the retrospective items was progressively questionable when presumed to cover longer time periods. CONCLUSIONS: Substantial measurement error was evident when identifying 'occasional-only' and 'never' drinkers using retrospective items covering the lifecourse. Researchers investigating potential health benefits associated with moderate drinking need to incorporate more sophisticated methods when identifying sub-groups of non-drinkers.     

Obesity, gestational weight gain and preterm birth: a study within the Danish National Birth Cohort

The aim of the present study was to assess the impact of obesity and gestational weight gain on the risk of subtypes of preterm birth, because little is known about these associations. The study included 62 167 women within the Danish National Birth Cohort for whom self-reported information about prepregnancy body mass index (BMI) and gestational weight gain was available. Information about spontaneous preterm birth with or without preterm premature rupture of membranes (PPROM) and about induced preterm deliveries was obtained from national registers. Cox regression analyses were used to examine associations of prepregnancy BMI and gestational weight gain with subtypes of preterm birth. The crude risks of PPROM and of induced preterm deliveries were higher in obese women (BMI > or = 30) than in normal-weight women (18.5 < or = BMI < 25), especially before 34 completed weeks of gestation, when obese women faced twice the risk. In the adjusted analysis, the hazard ratios and 95% confidence intervals for PPROM and for induced preterm delivery in obese women were 1.5 [1.2, 1.9] and 1.2 [1.0, 1.6] respectively. When obesity-related diseases were accounted for, no excess risk of induced preterm deliveries was seen in obese women, but the increased risk of PPROM was unchanged. Low weight gain was associated with an increased risk of all subtypes of preterm birth compared with normal weight gain, especially in early spontaneous preterm births, where the risk was doubled. We concluded that prepregnancy obesity was associated with a higher risk of PPROM and early induced preterm deliveries.     

Long-term outcome of children who were prematurely born or small for gestational age

All children born between 1971 and 1974 at the Haguenau maternity center (Lower-Rhine department, N.E. France) and attending school in Haguenau were examined by school doctors. The paper shows that there were differences in development of the 3 groups studied: 54 children born before term, 62 born small-for-dates and 1,188 controls. The rapid growth rate observed in the preterm children enabled them to catch up with the controls in physical growth criteria, in contrast to those in the small-for-dates group. Overall, those in the preterm group seemed to enjoy better health than those in the small-for-dates group, but their adaptation to school life was poorer.     

Work in pregnancy and birth weight for gestational age

In a recent report on prematurity and work in pregnancy based on the Montreal survey noteworthy increases in both preterm births (less than 37 weeks) and infants of low birth weight (less than or equal to 2500 g) were found in women in certain specific occupations or whose work entailed heavy lifting, shift work, long hours, or great fatigue. Because of the large overlap between preterm births and low birth weight, the latter was further analysed with allowance for gestational age in order better to separate factors retarding fetal growth from those shortening gestation. The association of low birth weight with specific occupations, long working hours, and fatigue largely disappeared, suggesting that the effect of these factors was to shorten gestation. By contrast, the association with lifting heavy weights and with shift work persisted, suggesting that these factors retarded fetal growth as well as increasing the risk of preterm birth.     

Risk of intellectual disability in children born appropriate-for-gestational-age at term or post-term: impact of birth weight for gestational age and gestational age

European Journal of Epidemiology - Children born small for gestational age have a higher risk of intellectual disability. We investigated associations of birth weight for gestational age percentile...     

On the pitfalls of adjusting for gestational age at birth

Preterm delivery is a powerful predictor of newborn morbidity and mortality. Such problems are due to not only immaturity but also the pathologic factors (such as infection) that cause early delivery. The understanding of these underlying pathologic factors is incomplete at best. To the extent that unmeasured pathologies triggering preterm delivery also directly harm the fetus, they will confound the association of early delivery with neonatal outcomes. This, in turn, complicates studies of newborn outcomes more generally. When investigators analyze the association of risk factors with neonatal outcomes, adjustment for gestational age as a mediating variable will lead to bias. In the language of directed acyclic graphs, gestational age is a collider. The theoretical basis for colliders has been well described, and gestational age has recently been acknowledged as a possible collider. However, the impact of this problem, as well as its implications for perinatal research, has not been fully appreciated. The authors discuss the evidence for confounding and present simulations to explore how much bias is produced by adjustments for gestational age when estimating direct effects. Under plausible conditions, frank reversal of exposure-outcome associations can occur. When the purpose is causal inference, there are few settings in which adjustment for gestational age can be justified.     

Birth weight and perinatal mortality: the effect of gestational age.

BACKGROUND. The strong association between birth weight and perinatal mortality is due both to gestational age and to factors unrelated to gestational age. Conventional analysis obscures these separate contributions to perinatal mortality, and overemphasizes the role of birth weight. An alternative approach is used here to separate gestational age from other factors. METHODS. Data are from 400,000 singleton births in the Norwegian Medical Birth Registry. The method of Wilcox and Russell is used to distinguish the contributions to perinatal mortality made by gestational age and by relative birth weight at each gestational age. RESULTS. Gestational age is a powerful predictor of birth weight and perinatal survival. After these effects of gestational age are controlled for, relative birth weight retains a strong association with survival. CONCLUSIONS. Current public health policies in the United States emphasize the prevention of low birth weight. The present analysis suggests that the prevention of early delivery would benefit babies of all birth weights.     

A birth weight for gestational age standard based on data in the Swedish Medical Birth Registry, 1985–1989

Birth weight curves according to gestational length are presented based on data from more than 480,000 singleton births, registered in the Medical Birth Registry and with gestational age based on ultrasound examinations in the majority of cases. Curves were constructed from the most common weights in each week (modes) for each sex and standard deviations were estimated under the assumption of a fixed coefficient of variation, the size of which was estimated from 40 weeks infants. This methodology makes it possible to construct graphs for specific subgroups of infants: such graphs for boys and girls for primiparous and multiparous women are given in the paper.     

Early environment and child-to-adult growth trajectories in the 1958 British birth cohort

BACKGROUND: Genetics and environmental conditions early in life are known to influence height. However, evidence is restricted to studies conducted at a specific age, and thus the effect on the entire growth trajectory has been neglected. OBJECTIVE: The objective was to determine when parental height and factors early in offspring life start to affect offspring height, when these variables have the strongest effect, and whether these variables persist to adulthood. DESIGN: Longitudinal data from the 1958 British birth cohort (all of whom were born during 1 wk in March 1958), including height measurements at 7, 11, 16, and 33 y of age, were analyzed by using multivariate multilevel response models. RESULTS: Parental height, birth weight, maternal smoking during pregnancy, breastfeeding, parental divorce, and socioeconomic factors were all significantly associated with childhood height, but their effects differed thereafter. Parental height and birth weight were most strongly associated with offspring height, and their effects persisted (adjusted increase in adult height: 2 cm for 1 SD of maternal or paternal height, or 1 kg of birth weight). Socioeconomic disadvantage (manual social class, large family size, and overcrowded households) was associated with substantial deficits of 2-3 cm (adjusted estimates) in height at 7 y. Catch-up growth was apparent but was insufficient to overcome the initial insult on growth; the adjusted deficit was as high as 1 cm in adulthood. CONCLUSIONS: Children from disadvantaged backgrounds have a delayed pattern of growth before the pubertal spurt, which is followed by catch-up growth. The health consequences of this pattern of growth need to be examined in future studies.     

Alcohol drinking and risk of small for gestational age birth

OBJECTIVE: To assess if alcohol drinking is a risk factor for small for gestational age (SGA) birth. METHODS: Case-control study. Cases were 555 women (mean age 31 years, range 16-43) who delivered SGA babies at the Clinica Luigi Mangiagalli and the Obstetric and Gynecology Clinic of the University of Verona. The controls were 1966 women (mean age 31 years, range 14-43) who gave birth at term (> or =37 weeks of gestation) to healthy infants of normal weight at the hospitals where cases had been identified. RESULTS: No increase in the risk of SGA birth was observed in women drinking one or two drinks/day in pregnancy, but three or more per day increased the risk: odds ratios (OR) were 3.2 (1.7-6.2) for > or =3 drinks during the first trimester, 2.7 (1.4-5.3) during the second and 2.9 (1.5-5.7) during the third. CONCLUSIONS: The study shows an increased risk of SGA births in mothers who drink > or =3 units/day of alcohol in pregnancy.