Birth weight and cognitive ability in childhood among siblings and nonsiblings

Small-for-Gestational-Age and Vocabulary and Achievement Test Scores at Age 9 Among Children Born at Term in a Contemporary U.S. Sample

INTRODUCTION

Children that are small-for-gestational-age (SGA) at birth are at an increased risk for cognitive impairment, even if born at term (37-41 weeks). This study examined associations between sex-specific SGA and vocabulary and achievement tests in 9 year old children born at term using a contemporary population-based US sample.

METHODS

A secondary data analysis was conducted on a sample of 2144 children born at term in 1998-2000 who participated in a US birth cohort study that oversampled non-marital births, which in the U.S. are associated with socioeconomic disadvantage and racial minority status. Vocabulary and achievement tests were administered to participants at age 9. Unadjusted and adjusted Ordinary Least Squares and logistic regression models of associations between SGA and test scores were estimated.

RESULTS

Sex-specific SGA was associated with 2-5 point lower test scores and 1-2 times the odds of scores less than 85 (> 1 SD below the national mean) across most outcomes. In adjusted models, measures of SGA were associated with low scores on the Woodcock-Johnson Applied Problems test (OR 2.257; 95% CI 1.434, 3.551) and the Woodcock-Johnson Passage Comprehension test (OR 1.554; 95% CI 1.132, 2.134).

CONCLUSION

The findings validate previous studies of SGA at term and cognitive outcomes and provide further evidence using a contemporary high-risk population-based US sample. The findings suggest that SGA children born at term should be recruited for early interventions to promote improved cognitive functioning in school.

Using Mendelian Randomisation to Prioritise Candidate Maternal Metabolic Traits Influencing Offspring Birthweight

Marked physiological changes in pregnancy are essential to support foetal growth; however, evidence on the role of specific maternal metabolic traits from human studies is limited. We integrated Mendelian randomisation (MR) and metabolomics data to probe the effect of 46 maternal metabolic traits on offspring birthweight (N = 210,267). We implemented univariable two-sample MR (UVMR) to identify candidate metabolic traits affecting offspring birthweight. We then applied two-sample multivariable MR (MVMR) to jointly estimate the potential direct causal effect for each candidate maternal metabolic trait. In the main analyses, UVMR indicated that higher maternal glucose was related to higher offspring birthweight (0.328 SD difference in mean birthweight per 1 SD difference in glucose (95% CI: 0.104, 0.414)), as were maternal glutamine (0.089 (95% CI: 0.033, 0.144)) and alanine (0.137 (95% CI: 0.036, 0.239)). In additional analyses, UVMR estimates were broadly consistent when selecting instruments from an independent data source, albeit imprecise for glutamine and alanine, and were attenuated for alanine when using other UVMR methods. MVMR results supported independent effects of these metabolites, with effect estimates consistent with those seen with the UVMR results. Among the remaining 43 metabolic traits, UVMR estimates indicated a null effect for most lipid-related traits and a high degree of uncertainty for other amino acids and ketone bodies. Our findings suggest that maternal gestational glucose and glutamine are causally related to offspring birthweight.

Association of maternal circulating 25(OH)D and calcium with birth weight: A mendelian randomisation analysis

BACKGROUND

Systematic reviews of randomised controlled trials (RCTs) have suggested that maternal vitamin D (25[OH]D) and calcium supplementation increase birth weight. However, limitations of many trials were highlighted in the reviews. Our aim was to combine genetic and RCT data to estimate causal effects of these two maternal traits on offspring birth weight.

METHODS AND FINDINGS

We performed two-sample mendelian randomisation (MR) using genetic instrumental variables associated with 25(OH)D and calcium that had been identified in genome-wide association studies (GWAS; sample 1; N = 122,123 for 25[OH]D and N = 61,275 for calcium). Associations between these maternal genetic variants and offspring birth weight were calculated in the UK Biobank (UKB) (sample 2; N = 190,406). We used data on mother-child pairs from two United Kingdom birth cohorts (combined N = 5,223) in sensitivity analyses to check whether results were influenced by fetal genotype, which is correlated with the maternal genotype (r ≈ 0.5). Further sensitivity analyses to test the reliability of the results included MR-Egger, weighted-median estimator, 'leave-one-out', and multivariable MR analyses. We triangulated MR results with those from RCTs, in which we used randomisation to supplementation with vitamin D (24 RCTs, combined N = 5,276) and calcium (6 RCTs, combined N = 543) as an instrumental variable to determine the effects of 25(OH)D and calcium on birth weight. In the main MR analysis, there was no strong evidence of an effect of maternal 25(OH)D on birth weight (difference in mean birth weight -0.03 g [95% CI -2.48 to 2.42 g, p = 0.981] per 10% higher maternal 25[OH]D). The effect estimate was consistent across our MR sensitivity analyses. Instrumental variable analyses applied to RCTs suggested a weak positive causal effect (5.94 g [95% CI 2.15-9.73, p = 0.002] per 10% higher maternal 25[OH]D), but this result may be exaggerated because of risk of bias in the included RCTs. The main MR analysis for maternal calcium also suggested no strong evidence of an effect on birth weight (-20 g [95% CI -44 to 5 g, p = 0.116] per 1 SD higher maternal calcium level). Some sensitivity analyses suggested that the genetic instrument for calcium was associated with birth weight via exposures that are independent of calcium levels (horizontal pleiotropy). Application of instrumental variable analyses to RCTs suggested that calcium has a substantial effect on birth weight (178 g [95% CI 121-236 g, p = 1.43 × 10-9] per 1 SD higher maternal calcium level) that was not consistent with any of the MR results. However, the RCT instrumental variable estimate may have been exaggerated because of risk of bias in the included RCTs. Other study limitations include the low response rate of UK Biobank, which may bias MR estimates, and the lack of suitable data to test whether the effects of genetic instruments on maternal calcium levels during pregnancy were the same as those outside of pregnancy.

CONCLUSIONS

Our results suggest that maternal circulating 25(OH)D does not influence birth weight in otherwise healthy newborns. However, the effect of maternal circulating calcium on birth weight is unclear and requires further exploration with more research including RCT and/or MR analyses with more valid instruments.

Gestational age at birth and academic performance: population-based cohort study

Background

Numerous studies suggest pre-term birth is associated with cognitive deficit. However, less is known about cognitive outcomes following post-term birth, or the influence of weight variations within term or post-term populations. We examined associations between gestational age (GA) and school performance, by weight-for-GA, focusing on extremely pre- and post-term births.

Method

Record linkage study of Swedish children born 1973-94 ( n =  2 008 102) with a nested sibling comparison ( n =  439 629). We used restricted cubic regression splines to examine associations between GA and the grade achieved on leaving secondary education, comparing siblings to allow stronger causal inference with regard to associations between GA and school performance.

Results

Grade averages of both pre- and post-term children were below those of full-term counterparts and lower for those born small-for-GA. The adjusted grades of extremely pre-term children (at 24 completed weeks), while improving in later study periods, were lower by 0.43 standard deviations (95% confidence interval 0.38-0.49), corresponding with a 21-point reduction (19 to 24) on a 240-point scale. Reductions for extremely post-term children (at 45 completed weeks) were lesser [-0.15 standard deviation (-0.17 to -0.13) or -8 points (-9 to -7)]. Among matched siblings, we observed weaker residual effects of pre-term and post-term GA on school performance.

Conclusions

There may be independent effects of fetal maturation and fetal growth on school performance. Associations among matched siblings, although attenuated, remained consistent with causal effects of pre- and post-term birth on school performance.

The association of high birth weight with intelligence in young adulthood: a cohort study of male siblings

We aimed to explore why, in population studies, the positive association between normal-range birth weight and intelligence becomes negative at the highest birth weights. The study population comprised 217,746 Norwegian male singletons born at term between 1967 and 1976. All had data on birth weight and intelligence quotient (IQ) score at the time of military conscription; 137,574 had data on sibling birth weights; and 62,906 had data on male sibling birth weights. We estimated associations between birth weight and IQ score by ordinary least squares regression for the total study population and by fixed-effects regression for comparisons of brothers. The crude mean IQ score was 1.2 points (95% confidence interval (CI): 0.3, 2.2) lower for those with birth weights of 5,000 g or more compared with the reference group (with birth weights of 4,000-4,499 g). This difference leveled off to 0.0 (95% CI: -0.8, 0.9) in multivariable ordinary least squares regression and reversed to 2.2 points (95% CI: 0.3, 4.2) higher in fixed-effects regression. Results differed mainly because, at a given birth weight, participants who had a sibling with macrosomia had a lower mean IQ score. Nevertheless, within families with 1 or more macrosomic siblings, as in other families, men with higher birth weights tended to have higher IQ scores. Thus, a family-level confounder introduces a cross-level bias that cannot be detected in individual-level studies. We suggest ways in which future studies might elucidate the nature of this confounder.

Approaches for drawing causal inferences from epidemiological birth cohorts: a review

Large-scale population-based birth cohorts, which recruit women during pregnancy or at birth and follow up their offspring through infancy and into childhood and adolescence, provide the opportunity to monitor and model early life exposures in relation to developmental characteristics and later life outcomes. However, due to confounding and other limitations, identification of causal risk factors has proved challenging and published findings are often not reproducible. A suite of methods has been developed in recent years to minimise problems afflicting observational epidemiology, to strengthen causal inference and to provide greater insights into modifiable intra-uterine and early life risk factors. The aim of this review is to describe these causal inference methods and to suggest how they may be applied in the context of birth cohorts and extended along with the development of birth cohort consortia and expansion of "omic" technologies.

Birth weight and cognition in children with epilepsy

OBJECTIVE

Birth weight is an important indicator of prenatal environment, and subtle variations of birth weight within the normal range have been associated with differential risk for cognitive and behavioral problems. Therefore, we aimed to determine if there are differences in birth weight between full-term children with uncomplicated new/recent-onset epilepsies and typically developing healthy controls. We further examined the relationships between birth weight and childhood/adolescent cognition, behavior, and academic achievement.

METHODS

One hundred eight children with new-onset/recent-onset epilepsy and 70 healthy controls underwent neuropsychological assessment. All participants were born full-term (>37 weeks) without birth complications. Parents were interviewed regarding their child's gestation, birth, and neurodevelopmental history.

RESULTS

Birth weight of children with epilepsy was significantly lower than healthy controls (p = 0.023). Whereas birth weight (covaried with age, sex, handedness, and mother's education) was significantly associated with cognition in controls in multiple domains (intelligence, language, aspects of academic achievement), this relationship was absent in children with epilepsy. Birth weight was not associated with clinical epilepsy variables (age of onset, epilepsy syndrome) and was not predictive of a variety of other academic or psychiatric comorbidities of epilepsy.

SIGNIFICANCE

Although the origin of lower birth weight in children with epilepsy is unknown, these findings raise the possibility that abnormal prenatal environment may affect childhood-onset epilepsy. Furthermore, the positive relationship between birth weight and cognition evident in healthy controls was disrupted in children with epilepsy. However, birth weight was not related to academic and psychiatric comorbidities of childhood epilepsy. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here.

The impact of prepregnancy obesity on children's cognitive test scores

To examine the association between maternal prepregnancy obesity and cognitive test scores of children at early primary school age. A descriptive observational design was used. Study subjects consist of 3,412 US children aged 60-83 months from the National Longitudinal Survey of Youth 1979 Mother and Child Survey. Cognitive test scores using the Peabody Individual Achievement Test reading recognition and mathematics tests were used as the outcomes of interest. Association with maternal prepregnancy obesity was examined using the ordinary least square regression controlling for intrauterine, family background, maternal and child factors. Children of obese women had 3 points (0.23 SD units) lower peabody individual achievement test (PIAT) reading recognition score (p = 0.007), and 2 points (0.16 SD units) lower PIAT mathematics scores (p < 0.0001), holding all other factors constant. As expected, cognitive test score was associated with stimulating home environment (reading: β = 0.15, p < 0.0001, and math: β = 0.15, p < 0.0001), household income (reading: β = 0.03, p = 0.02 and math: β = 0.04, p = 0.004), maternal education (reading: β = 0.42, p = 0.0005, and math: β = 0.32, p = 0.008), and maternal cognitive skills (reading: β = 0.11, p < 0.0001, and math: β = 0.09, p < 0.0001). There was a significant association between maternal prepregnancy obesity and child cognitive test scores that could not be explained by other intrauterine, family background, maternal, and child factors. Children who live in disadvantaged postnatal environments may be most affected by the effects of maternal prepregnancy obesity. Replications of the current study using different cohorts are warranted to confirm the association between maternal prepregnancy obesity and child cognitive test scores.

Associations of maternal weight gain in pregnancy with offspring cognition in childhood and adolescence: findings from the Avon Longitudinal Study of Parents and Children

An association of gestational weight gain (GWG) with offspring cognition has been postulated. We used data from the Avon Longitudinal Study of Parents and Children, a United Kingdom prospective cohort (1990 through the present) with a median of 10 maternal weight measurements in pregnancy. These were used to allocate participants to 2009 Institute of Medicine weight-gain categories and in random effect linear spline models. Outcomes were School Entry Assessment score (age, 4 years; n = 5,832), standardized intelligence quotient assessed by Wechsler Intelligence Scale for Children (age, 8 years; n = 5,191), and school final-examination results (age, 16 years; n = 7,339). Offspring of women who gained less weight than recommended had a 0.075 standard deviation lower mean School Entry Assessment score (95% confidence interval: -0.127, -0.023) and were less likely to achieve adequate final-examination results (odds ratio = 0.88, 95% confidence interval: 0.78, 0.99) compared with offspring of women who gained as recommended. GWG in early pregnancy (defined as 0-18 weeks on the basis of a knot point at 18 weeks) and midpregnancy (defined as 18-28 weeks on the basis of knot points at 18 and 28 weeks) was positively associated with School Entry Assessment score and intelligence quotient. GWG in late pregnancy (defined as 28 weeks onward on the basis of a knot point at 28 weeks) was positively associated with offspring intelligence quotient and with increased odds of offspring achieving adequate final-examination results in mothers who were overweight prepregnancy. Findings support small positive associations between GWG and offspring cognitive development, which may have lasting effects on educational attainment up to age 16 years.

Child height, health and human capital: Evidence using genetic markers

Height has long been recognized as being associated with better outcomes: the question is whether this association is causal. We use children's genetic variants as instrumental variables to deal with possible unobserved confounders and examine the effect of child/adolescent height on a wide range of outcomes: academic performance, IQ, self-esteem, depression symptoms and behavioral problems. OLS findings show that taller children have higher IQ, perform better in school, and are less likely to have behavioral problems. The IV results differ: taller girls (but not boys) have better cognitive performance and, in contrast to the OLS, greater height appears to increase behavioral problems.

Long-term influence of normal variation in neonatal characteristics on human brain development

It is now recognized that a number of cognitive, behavioral, and mental health outcomes across the lifespan can be traced to fetal development. Although the direct mediation is unknown, the substantial variance in fetal growth, most commonly indexed by birth weight, may affect lifespan brain development. We investigated effects of normal variance in birth weight on MRI-derived measures of brain development in 628 healthy children, adolescents, and young adults in the large-scale multicenter Pediatric Imaging, Neurocognition, and Genetics study. This heterogeneous sample was recruited through geographically dispersed sites in the United States. The influence of birth weight on cortical thickness, surface area, and striatal and total brain volumes was investigated, controlling for variance in age, sex, household income, and genetic ancestry factors. Birth weight was found to exert robust positive effects on regional cortical surface area in multiple regions as well as total brain and caudate volumes. These effects were continuous across birth weight ranges and ages and were not confined to subsets of the sample. The findings show that (i) aspects of later child and adolescent brain development are influenced at birth and (ii) relatively small differences in birth weight across groups and conditions typically compared in neuropsychiatric research (e.g., Attention Deficit Hyperactivity Disorder, schizophrenia, and personality disorders) may influence group differences observed in brain parameters of interest at a later stage in life. These findings should serve to increase our attention to early influences.

Does shared family background influence the impact of educational differences on early mortality?

The mechanisms behind social differences in mortality rates have been debated. The authors examined the extent to which shared family background and health in early life could explain the association between educational status and all-cause mortality rates using a sibling design. The study was register-based and included all individuals born in Denmark between 1950 and 1979 who had at least 1 full sibling born in the same time period (n = 1,381,436). All individuals were followed from 28 years of age until death, emigration, or December 2009. The authors used Cox regression analyses to estimate hazard ratios for mortality according to educational level. Conventional cohort and intersibling analyses were carried out and conducted separately for deaths occurring before and after the age of 45 years, respectively. The cohort analyses showed an inverse association between educational status and all-cause mortality that was strongest for males, increased with younger birth cohorts, and tended to be strongest in the analyses of death before 45 years of age. The associations were attenuated slightly in the intersibling analyses and after adjustment for serious health conditions in early life. Hence, health selection and confounding by factors shared by siblings explained only a minor part of the association between educational level and all-cause mortality.

The role of maternal smoking in effect of fetal growth restriction on poor scholastic achievement in elementary school

Fetal growth restriction and maternal smoking during pregnancy are independently implicated in lowering intellectual attainment in children. We hypothesized that only reduction of fetal growth that is attributable to extrinsic causes (e.g., maternal smoking) affects intellectual development of a child. Cross-sectional survey of 3,739 students in Nova Scotia (Canada) in 2003 was linked with the perinatal database, parental interviews on socio-demographic factors and the performance on standardized tests when primarily 11–12 years of age, thereby forming a retrospective cohort. Data was analyzed using hierarchical logistic regression with correction for clustering of children within schools. The risk of poor test result among children born small-for-gestational-age (SGA) to mothers who smoked was 29.4%, higher than in any other strata of maternal smoking and fetal growth. The adjusted odds ratio among SGA children born to mothers who smoked was the only one elevated compared to children who were not growth restricted and born to mothers who did not smoke (17.0%, OR = 1.46, 95% CI 1.02, 2.09). Other perinatal, maternal and socio-demographic factors did not alter this pattern of effect modification. Heterogeneity of etiology of fetal growth restriction should be consider in studies that address examine its impact on health over life course.

The effect of intrauterine growth on verbal IQ scores in childhood: a study of monozygotic twins

OBJECTIVE

Given the adverse neurobiological effects of suboptimal nutrition on the developing brain, it is of social and medical importance to determine if the global prevalence of poor intrauterine growth causes lasting cognitive deficits. We examined whether suboptimal intrauterine growth relates to impaired cognitive outcome by comparing birth weight and cognition in monozygotic twins and considered whether children within-pair differences in birth weight were related to within-pair differences in IQ scores.

METHODS

A total of 71 monozygotic twin pairs (aged 7 years 11 months to 17 years 3 months) participated. The Wechsler Intelligence Scale for Children, Third Edition, was administered, and verbal IQ (VIQ) and performance IQ (PIQ) scores were calculated. Regression was used to relate within-pair differences in birth weight to within-pair differences in IQ scores.

RESULTS

VIQ but not PIQ score was affected by prenatal growth restriction. The results suggest that the mean advantage for heavier twins relative to their lighter co-twins can be as much as half an SD in VIQ points. In pairs with minimal discordance, heavier twins had lower VIQ scores than their lighter co-twins.

CONCLUSIONS

Our study results suggest that lower birth weight in monozygotic twins can also have a negative long-term impact on cognition both in infants who are small at birth and also those with birth weights across the spectrum. Studying monozygotic twins enabled us to examine the effect of reduced intrauterine growth on cognition independently of confounding factors, including parental IQ and education and infant gender, age, genetic characteristics, and gestation.

Birth weight standardized to gestational age and intelligence in young adulthood: a register-based birth cohort study of male siblings

The authors aimed to determine the relation between birth-weight variations within the normal range and intelligence in young adulthood. A historical birth cohort study was conducted. Data from the Medical Birth Register of Norway were linked with register data from the Norwegian National Conscript Service. The sample comprised 52,408 sibships of full brothers who were born singletons at 37-41 completed weeks' gestation during 1967-1984 in Norway and were intelligence-tested at the time of mandatory military conscription. Generalized estimating equations were used to fit population-averaged panel data models. The analyses showed that in men with birth weights within the 10th-90th percentile range, a within-family difference of 1 standard deviation in birth weight standardized to gestational age was associated with a within-family difference of 0.07 standard deviation (99% confidence interval: 0.03, 0.09) in intelligence score, after adjustment for a range of background factors. There was no significant between-family association after adjustment for background factors. In Norwegian males, normal variations in intrauterine growth are associated with differences in intelligence in young adulthood. This association is probably not due to confounding by familial and parental characteristics.

Gestational age differences in health and development among young Swedish men born at term

BACKGROUND

Although increased morbidity and mortality associated with pre-term birth and restricted fetal growth have been extensively studied, relatively little is known about variations in health outcomes among term births, because they are often assumed to be homogeneous.

METHODS

We examined variations in height, body mass index (BMI), systolic and diastolic blood pressure (SBP and DBP), and intellectual performance by gestational age and fetal 'growth' (birth weight for gestational age) among young Swedish men born at term (37-41 weeks of gestation). We also compared the magnitude of associations among 314,642 men from different families and among 72,212 full brothers from 35,215 families to assess whether the associations are explained by familial factors shared by siblings.

RESULTS

Gestational age in completed weeks was positively associated with height [0.11 cm, 95% confidence interval (CI): 0.09-0.13] and intellectual performance (0.01, 95% CI: 0.00-0.02) and negatively associated with SBP (-0.28 mmHg, 95% CI: -0.33 to -0.24), after controlling for birth weight, maternal age at the men's birth, parity, family socio-economic position and family structure. The associations with height and SBP were observed also among brothers within families, suggesting that they are not explained by shared family characteristics. However, the positive association between gestational age and intellectual performance was no longer present within families. Birth weight for gestational age (z-score) was positively associated with height, BMI and intellectual performance and negatively associated with SBP. These associations were robust within families.

CONCLUSIONS

Among young men born at term, fetal growth and even gestational age are independently associated with adult size, BP and cognitive ability. The extent to which shared family characteristics explain the associations varies across outcomes.

Variation in child cognitive ability by week of gestation among healthy term births

The authors investigated variations in cognitive ability by gestational age among 13,824 children at age 6.5 years who were born at term with normal weight, using data from a prospective cohort recruited in 1996-1997 in Belarus. The mean differences in the Wechsler Abbreviated Scales of Intelligence were examined by gestational age in completed weeks and by fetal growth after controlling for maternal and family characteristics. Compared with the score for those born at 39-41 weeks, the full-scale intelligence quotient (IQ) score was 1.7 points (95% confidence interval (CI): -2.7, -0.7) lower in children born at 37 weeks and 0.4 points (95% CI: -1.1, 0.02) lower at 38 weeks after controlling for confounders. There was also a graded relation in postterm children: a 0.5-points (95% CI: -2.6, 1.6) lower score at 42 weeks and 6.0 points (95% CI: -15.1, 3.1) lower at 43 weeks. Compared with children born large for gestational age (>90th percentile), children born small for gestational age (<10th percentile) had the lowest IQ, followed by those at the 10th-50th percentile and those at the >50th-90th percentile. These findings suggest that, even among healthy children born at term, cognitive ability at age 6.5 years is lower in those born at 37 or 38 weeks and those with suboptimal fetal growth.