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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.

The risk of short adult stature in women born small-for-gestational age (SGA) was estimated in this prospective cohort study of 43 872 singleton females, born between 1973 and 1983, who gave birth to a child between 1989 and 1999. The risk of overweight in females born SGA, with and without short adult stature, was also studied. All data on birth characteristics and adult height and weight were obtained from the Swedish Birth Register. SGA-born females were divided into being born short only for gestational age [birth length < -2 standard deviation scores (SDS)], born light for gestational age (birthweight < -2 SDS) or being born both short and light for gestational age. Short adult stature was defined as adult height below -2 SDS. Among females, being born SGA (<-2 SDS in birth length or birthweight) was associated with increased risk of short adult stature, compared with being born appropriate for gestational age. The risk varied substantially within different subgroups of females born SGA: being born short for gestational age was associated with an almost fivefold increased risk [odds ratio (OR) 4.89; 95% confidence interval (CI) 3.70, 6.47] of short adult stature, whereas being born light for gestational age was associated with an almost twofold increased risk [OR 1.95, 95% CI 1.43, 2.65]. Overall, females born SGA did not have increased risk of overweight compared with females with appropriate size at birth. However, among females born short for gestational age, short adult stature was associated with an increased risk of overweight in adulthood [OR 1.77, 95% CI 1.01, 3.12]. In conclusion, among females born SGA, of the birth characteristics, short birth length is associated with the highest increased risk of short adult stature. Spontaneous growth in height to normal adult stature reduced the risk of overweight in females born short for gestational age.

All male singletons born without congenital malformations in Sweden between 1973 and 1978 and conscripted between January 1991 and January 1997 (n = 254,426) were studied. Intellectual and psychological performance was tested at conscription. Males born small for gestational age (SGA) had lower results on both intellectual and psychological performance testing. Among males born SGA, low mean scores in both tests were constantly more common in those without catch-up growth than in those with catch-up growth. In conclusion, being born SGA is associated with an increased risk of subnormal intellectual and psychological performance. Catch-up growth is associated with a reduced risk of subnormal performance in males born SGA.