Low birthweight and risk of mild mental retardation by ages 5 and 9 to 11.

This prospective analysis assessed the risk of mild mental retardation (MMR) associated with low birthweight (LBW) in the Child Health and Development Studies. Scores of 50-70 on the Raven Progressive Matrices, a relatively culture-free test of cognitive functioning, were used to categorize MMR. At the age of 5, 13.8% of the 195 children with birthweights less than 2500 g (LBW) were MMR, whilst 4.2% of the 2293 children with normal birthweights (> 2955 grams) were MMR. After adjusting for confounders (maternal age, race, education, prenatal alcohol use, maternal conditions, and congenital anomalies), the relative risk of MMR for LBW was 3.4 (95% CI 1.2-5.4). For children aged 9-11, 7.7% of 194 LBW children were MMR, compared with 6.2% of the 2546 with normal birthweights; the adjusted relative risk for LBW was 1.2 (95% CI 0.7-2.0). Although a strong association between LBW and MMR was observed for both blacks and non-blacks at the age of 5, the association between birthweight and MMR was apparent only for blacks in the cohort of children aged 9-11. These findings suggest that race, a marker for environmental factors which were not measured in this study, may modify the LBW and MMR relationship.