No relationship between birth season and age at menarche in Indonesia

In utero conditions and menarche are two important themes in human development, so the influence of birth season on menarche is of great interest. We investigated the relationship between birth season and age at menarche in the tropics by analysing a nationally representative sample of an Islamic population with a large sample size. We extracted 10,190 ever-married women, mostly aged 15-49, from the Indonesian Family Life Survey (IFLS) and IFLS East. We regressed age at menarche on birth month and subsequently birth season, while controlling for province fixed effects, current age fixed effects, and education. When we controlled only for province fixed effects, only women born in August exhibited a statistically significant difference in age at menarche (0.16 years later) relative to those born in January. Controlling for more covariates made the difference statistically nonsignificant. Birth season was not statistically significantly related to age at menarche, regardless of the set of covariates. The same was true when we focused on rural, poor women, presumably most affected by seasonality. Given inconsistent results in the literature and our null relationship between birth month or season and age at menarche, we doubt that there is any relationship between birth season and age at menarche.