For both cohorts, we considered the same set of socioeconomic and biological factors as potential confounders: maternal and paternal education, maternal smoking status, maternal employment status, household income, sex of the child, maternal parity, and paternal age. We first did not include preterm births and birthweight in our statistical model because these factors are likely to be on the causal pathway between maternal age and child health (i.e. they may be considered intermediary variables) . In the follow-up analyses, however, we additionally included preterm births and birthweight in our analyses to account for their potentially mediating effects. We also examined whether maternal parity (i.e., the experience of having more children) influences the relationship between maternal age and child health via increased knowledge of infant care.