Influence of variation in birth weight within normal range and within sibships on IQ at age 7 years: cohort study

Abstract

Objective To examine the relation between birth weight and measured intelligence at age 7 years in children within the normal range of birth weight and in siblings. Design Cohort study of siblings of the same sex. Setting 12 cities in the United States. Subjects 3484 children of 1683 mothers in a birth cohort study during the years 1959 through 1966. The sample was restricted to children born at >37 weeks gestation and with birth weights of 1500-3999 g. Main outcome measure Full scale IQ at age 7 years. Results Mean IQ increased monotonically with birth weight in both sexes across the range of birth weight in a linear regression analysis of one randomly selected sibling per family (n = 1683) with adjustment for maternal age, race, education, socioeconomic status, and birth order. Within same sex sibling pairs, differences in birth weight were directly associated with differences in IQ in boys (812 pairs, predicted IQ difference per 100 g change in birth weight = 0.50, 95% confidence interval 0.28 to 0.71) but not girls (871 pairs, 0.10, − 0.09 to 0.30). The effect in boys remained after differences in birth order, maternal smoking, and head circumference were adjusted for and in an analysis restricted to children with birth weight > 2500 g. Conclusion The increase in childhood IQ with birth weight continues well into the normal birth weight range. For boys this relation holds within same sex sibships and therefore cannot be explained by confounding from family social environment.

Childhood intelligence, educational attainment and adult body mass index: findings from a prospective cohort and within sibling-pairs analysis

“…Indeed in other within family studies the lack of any effect within siblings, when an effect is seen between families, is generally interpreted as demonstrating confounding by socioeconomic position. [17][18][19] However, the fact that adjustment for father's occupational social class had no effect on the association between education and BMI in the whole cohort suggests that this may not be the most important family-based characteristic that explains this association. Other factors, such as parental education and intelligence, neighbourhood-and schoolbased characteristics that are similar for siblings are also likely to be important.…”

“…Such sibling studies are increasingly used in life course epidemiology. They have been used to examine the extent of confounding by parental and family factors in the associations of birth weight with childhood intelligence [17][18][19][20] and of birth weight with adult blood pressure. 21 However, we are unaware of any such sibling study in the assessment of the association between childhood intelligence/educational attainment and adult BMI.…”

Effect of placental function on fatty acid requirements during pregnancy

“…Observational studies correlating cord blood DHA status at birth and visual and intellectual function either suggest that there is no link (Bakker et al, 1999;Ghys et al, 2002) or they imply a negative association; it has been reported that DHA status is inversely proportional to birth weight (Rump et al, 2001), yet there is a wellestablished positive relationship between birth weight and IQ in the general population (Matte et al, 2001;Richards et al, 2001). However, such observational studies have to be interpreted with caution as cord blood levels are actually the net result of placental supply and fetal utilisation and may not correlate exactly with fetal availability.…”

Seasonal fluctuations in birth weight and neonatal limb length; does prenatal vitamin D influence neonatal size and shape?

“…As detailed above, heavier birth weights are associated with increased intelligence [2]. While the current study cannot comment on whether or not the seasonal changes in size and shape persist into adult, other studies have reported an association between season of birth and adult height [19,51].…”

“…In addition to providing insights into prenatal development, this variable is known to be associated with a wide range of important cognitive, behavioural and health outcomes in infancy, childhood and adulthood. For example, even within the normal range of birth weights, heavier birth weight has been associated with superior neurocognitive outcomes in several cohort studies [2][3][4][5][6]. There is accumulating evidence linking birth weight and wide range of chronic, adult-onset disorders [7,8].…”