Findings from previous studies have been inconsistent for associations with birth order and maternal age at birth.10-12,24,56 While the association with firstborn status was specific to very early menarche, we also found an association with early menarche for having had a teenage mother. Even though we adjusted associations for childhood family income, residual confounding may remain because teenage birth is related to poor socioeconomic status. The association with young maternal age at birth may be confounded by the maternal age at menarche because the mother’s menarche necessarily preceded her teenage pregnancy with her daughter, and it is known that ages at menarche are correlated for mothers and daughters.7 However, we have no data on the age at menarche of the participant’s mother. Our finding of an association between having been born from a multiple birth and very late age at menarche is consistent with a large UK cohort study that reported later age at menarche for twins.12
