Objective To examine the association between maternal age and birth outcomes, and to investigate the role of sociodemographic and lifestyle-related determinants.Design Population-based prospective cohort study from early pregnancy onwards.Setting Rotterdam, the Netherlands.Population A cohort of 8568 mothers and their children.Methods Maternal age was assessed at enrolment. Information about sociodemographic (height, weight, educational level, ethnicity, parity) and lifestyle-related determinants (alcohol consumption, smoking habits, folic acid supplement use, caffeine intake, daily energy intake) and birth outcomes was obtained from questionnaires and hospital records. Multivariate linear and logistic regression analyses were used.Main outcomes measures Birthweight, preterm delivery, smallfor-gestational-age, and large-for-gestational-age.Results As compared with mothers aged 30-34.9 years, no differences in risk of preterm delivery were found. Mothers younger than 20 years had the highest risk of delivering small-forgestational-age babies(OR 1.6, 95% CI: 1.1-2.5); however, this increased risk disappeared after adjustment for sociodemographic and lifestyle-related determinants. Mothers older than 40 years had the highest risk of delivering large-for-gestational-age babies (OR 1.3, 95% CI: 0.8-2.4). The associations of maternal age with the risks of delivering large-for-gestational-age babies could not be explained by sociodemographic and lifestyle-related determinants.Conclusions As compared with mothers aged 30-34.9 years, younger mothers have an increased risk of small-for-gestationalage babies, whereas older mothers have an increased risk of largefor-gestational-age babies. Sociodemographic and lifestyle-related determinants cannot fully explain these differences.

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