Early head growth: relation with IQ at 8 years and determinants in term infants of low and appropriate birthweight.

Aim: To investigate the relation between head growth at different periods and IQ at 8 years, and to identify factors associated with more rapid head growth.

Method: Two parallel cohorts of term low birthweight (LBW) and appropriate birthweight (ABW) infants were enrolled at birth in northeast Brazil. Anthropometric measurements were made at birth, 2 months, 6 months, 12 months, 24 months, and 8 years. Cognition was assessed at 8 years (n=164) with the Wechsler Intelligence Scale for Children. Multivariable analysis with a two-stage residual model was used to relate head growth between successive time points with IQ.

Results: Mean birthweight was 2.35kg (SD 0.15) in the LBW group (33 males, 50 females), and 3.21kg (SD 0.15) in the ABW group (28 males, 53 females). Mean gestational age was 38.8 weeks (SD 1.4) and 40.0 weeks (SD 1.3) respectively. In the LBW group, head growth from birth to 2 months and from 2 to 6 months, conditional on previous size, were significant independent predictors of IQ at 8 years. Conditional head growth from 6 months to 8 years and head size at birth were unrelated to IQ. In the ABW group there was no significant relation between conditional head growth and IQ for any period. Determinants of more rapid head growth from birth to 6 months in LBW infants were maternal height and rate of infant weight gain.

Interpretation: Head growth from birth to 6 months in term LBW infants is more important than prenatal or later postnatal head growth in predicting IQ at 8 years.