Intrauterine growth and intelligence within sibling pairs: findings from the Mater-University study of pregnancy and its outcomes.

OBJECTIVE To examine the association between intrauterine growth and intelligence. DESIGN Population based birth cohort study of sibling pairs born within a maximum of three years of each other. SETTING Mater-University women and children's hospital, Brisbane, Australia. PARTICIPANTS 235 (470 children) sibling pairs. RESULTS Among one randomly selected sibling from each pair verbal comprehension at age 5, general intelligence at age 14, and reading ability at age 14 increased linearly with increasing gestational age and sex standardised birth weight z scores. With adjustment for maternal age, race, and smoking during pregnancy, birth order, family income, and parental education the associations with verbal comprehension at age 5 and general intelligence at age 14 remained, whereas the association with reading ability at age 14 was attenuated to the null. Within sibling pairs, differences in intrauterine growth were positively associated with differences in verbal comprehension at age 5 (test score difference per one unit difference in birth weight z score = 1.52 (0.11 to 3.26)) and general intelligence at age 14 (1.09 (0.01 to 2.18)), but not with reading ability at age 14. CONCLUSIONS Socioeconomic position or other fixed maternal characteristics do not seem to explain the positive association between intrauterine growth and childhood intelligence.