Aim: To study the effect of size at birth on different dimensions of intellectual capacity.
Methods: The study comprised a population-based cohort including all male single births without congenital malformations in Sweden from 1973 to 1976, and conscripted before 1994 (n = 168 068). Information from the Swedish Birth Register was individually linked to the Swedish Conscript Register. The test of intellectual performance included four different dimensions: logical, spatial, theoretical and verbal capacity. These data were available for 80-86% of the males at conscription.
Results: Compared with boys born appropriate for gestational age, males born small for gestational age (SGA) had an increased risk for subnormal performance in all four dimensions. Among males born SGA who were also of short adult stature at conscription, and in individuals born SGA with a head circumference <-- 2 SDS at birth, the risk of subnormal performance was most marked in the logical dimension (OR 1.52; CI 1.25-1.84 and 1.33; 1.15-1.55, respectively).
Conclusions: Being born small for gestational age is associated with increased risk of subnormal capacity in all four dimensions of intellectual performance. In SGA males, short adult stature, or a small head circumference at birth is especially associated with the risk of subnormal logical performance.