Gestational age and cognitive ability in early childhood: a population-based cohort study.

BACKGROUND: Recent studies suggest that children born at late preterm (34-36 weeks gestation) and early term (37-38 weeks) may have poorer developmental outcomes than children born at full term (39-41 weeks). We examined how gestational age is related to cognitive ability in early childhood using the U.K. Millennium Cohort Study. METHODS: Cognitive development was assessed using Bracken School Readiness Assessment at age 3 years, British Ability Scales II at ages 3, 5 and 7 years and Progress in Mathematics at age 7 years. Sample size varied according to outcome between 12,163 and 14,027. Each gestational age group was compared with the full-term group using differences in z-scores and risk ratios for scoring more than -1 SD below the mean. RESULTS: Children born at <32 weeks gestation scored lower (P < 0.05) than the full-term group on all scales with unadjusted z-score differences ranging between -0.8 to -0.2 SD. In all groups, there was an increased risk (P < 0.05) of scoring less than -1 SD below the mean compared with the full-term group for some of the tests: those born at < 32 weeks had a 40-140% increased risk in seven tests, those born at 32-33 weeks had a 60-80% increased risk in three tests, those born at 34-36 weeks had a 30-40% increased risk in three tests, and those born at 37-38 weeks had a 20% increased risk in two tests. CONCLUSIONS: Cognitive ability is related to the entire range of gestational age, including children born at 34-36 and 37-38 weeks gestation.