Objective: We sought to study whether the association between being born small for gestational age and risk for low intellectual performance was modified by gestational age.
Methods: A population-based cohort study was conducted of 352,125 boys who were born between 28 and 43 completed weeks of gestation from 1973 to 1981 in Sweden. Risk for low intellectual performance at military conscription, estimated as odds ratios with 95% confidence intervals was measured.
Results: Compared with men who were born preterm (28-36 weeks) and had normal birth weight for gestational age, men who were born preterm and had a very low birth weight for gestational age were not at increased risk for low intellectual performance. In contrast, men who were born preterm with a very short birth length or a very small head circumference for gestational age faced a near doubled risk for low intellectual performance compared with their appropriate peers. Among men who were born at term (37-41 weeks), risk for low intellectual performance was increased among those with very or moderately small birth weight, birth length, or head circumference for gestational age.
Conclusions: During early stages of gestation, growth in length and head circumference may be more important for intellectual development than weight increase. Future studies on size at birth and intellectual performance should consider also including anthropometric measurements other than birth weight.