Prenatal sunlight in the second trimester predicts timing of menarche in Polish women born between 1985 and 1991.

Season of birth has long been associated with variation in the timing of sexual maturation. However, the specific prenatal environmental mechanisms underlying this relationship remain unclear. We aimed to examine the association between prenatal maternal sunshine exposure and age at menarche, accounting for season of birth and relevant biological and socioeconomic factors. The study included 1,635 Polish female university students aged 19-25 years. Age at menarche was retrospectively reported. Prenatal sunshine exposure was estimated using national meteorological data on average monthly sunshine duration in Poland and calculated separately for each trimester of pregnancy. Generalized linear models assessed associations between trimester-specific sunshine exposure, season of birth, and age at menarche, with progressive adjustment for birthweight, parental education, urbanization level, number of siblings, and birth order. Sequential multiple regression analyses evaluated independent trimester effects. The mean age at menarche was 12.70 ± 1.17 years. In unadjusted analyses, summer and autumn birth were associated with earlier menarche; however, these associations were fully attenuated after adjusting for prenatal sunshine exposure. Higher maternal sunshine exposure during the second trimester was consistently and robustly associated with earlier menarche across all models (e.g. for the final model β=-0.145, 95% CI -0.21, -0.08; adj. R