Birth length is perhaps a better predictor of adult height and weight than birth weight, and should be considered as a possible risk factor for adult morbidity and mortality.

ABSTRACT:The influences of prenatal and postnatal growth on intellectual performance are unclear. We examined the associations of birth size and gestational age with intellectual performance and explored whether these associations were influenced by adult body size and social factors. In this nationwide cohort study, the records of 317,761 male infants registered in the Medical Birth Registry of Norway (1967Norway ( -1979 were linked to the Norwegian Conscript Service (1984Service ( -1999. The variation in intelligence test score at age 18 due to birth weight and birth length was evaluated using absolute and standardized (z scores) values. Mean intelligence score increased by gestational age, birth weight, and birth length. However, a decline in intellectual performance was observed for gestational age Ͼ41 wk and birth weight Ͼ4500 g. There was a strong interaction on intellectual performance between birth size and gestational age (p Ͻ 0.0005). Adjusting for adult size strongly attenuated the association of birth size with intellectual performance. The overall R 2 of intellectual performance explained by birth size was Ͻ1%; however, adding adult body size and social factors to the model increased R 2 to 12%. In conclusion, the association of birth size with intellectual performance was weak, but still present after adjustment for adult body size and social factors. I ntellectual performance appears to be negatively associated with important health outcomes such as all-cause mortality (1-4) and cardiovascular disease (1). Developmental research has provided evidence that insults during critical periods in early life may have lifetime consequences (5). Numerous epidemiologic studies have reported that perinatal factors including birth weight, birth length, and gestational age may influence later intellectual performance, but the mechanisms for these associations are unclear (6 -19). Understanding the early life influences on intellectual performance may be important for developing initiatives aimed at preventing adverse outcomes in later life.The association between birth size and intellectual performance may be wholly or partly explained by later growth rather than perinatal factors (20) because birth size is known to be directly related to adult height (21) and a positive correlation between adult height and intelligence has been reported (6,10,22-25). To our knowledge, only one study has so far examined the independent effects of birth weight and current size on intelligence, but this study did not include gestational age (10). Furthermore, only a small number of population-based studies exists on the association between birth weight and intelligence (18), and of these studies, the largest did not adjust for socioeconomic status (26).To assess the associations of birth weight, birth length, and gestational age with adult intellectual performance and to explore whether the associations were influenced by adult body size and social factors, we conducted a historical birth cohort study following a large sample of...

The authors discuss how the sibship design can be used to detect and control for familial confounding. Family-level confounding is especially problematic when estimating modest individual-level effects in the presence of familial confounders with large effects. This circumstance arises frequently in studies which relate indicators of fetal growth, such as birth weight, to outcomes that are strongly associated with parental socioeconomic status and genes. The study by Eriksen et al. in this issue of the Journal (Am J Epidemiol. 2010;172(5):530-536) uses the sibship design to capture the relation between birth weight, gestational age, and intelligence score among Norwegian males born as singletons at 37-41 completed weeks' gestation during 1967-1984. Their study illustrates how valuable the design can be in this kind of scenario. It also illustrates the potential complexity of sibship studies and the challenges they present for appropriate interpretation.

Infants born with birth defects have poorer outcomes in terms of mortality and disability, but the long-term intellectual outcome in children with birth defects is generally unknown. We assessed the long-term associations of various birth defects with mortality and disability, and evaluated whether high mortality and disability were reflected in impaired intellectual performance at age 18. In this nationwide cohort study, records of 9,186 males with and 384,384 without birth defects, registered in the Medical Birth Registry of Norway (1967Norway ( -1979 were linked to the National Conscript Service (1984 -1999). Mortality and disability before military draft, and intelligence test score at conscription were the main outcome measures. Males with birth defects had a relative risk for disability of 6.0 compared with males without defects. Disability was low within categories of birth defects associated with low mortality, and high within defect categories associated with high mortality. The relative risk for not being drafted was highest if maternal educational level was low. Heart defects and cleft palate were the only subgroups in which intellectual performance was lower after adjustment for maternal education, maternal age, marital status and birth order. In particular, intellectual performance was not impaired among those with multiple compared with single defects. We conclude that for the majority of birth defect categories in the present birth cohort, our hypothesis that intellectual performance would be impaired was not confirmed. Thus, there seems to be little reason to fear an adverse intellectual outcome in non-disabled surviving infants with birth defects. (Pediatr Res 59: 848-853, 2006)