Seasonal rhythms of menarche in the United States: Correlates to menarcheal age, birth age, and birth month

Purpose: To examine the seasonality of menarche in a large sample of women from the United States. Methods: Retrospective data were collected on month of birth and menarche, age of menarche, and latitude and altitude of residence from a sample of approximately 3,000 U.S. women from the TREMIN Research Program and from a college student population. Results: Monthly analyses using observed (0) to expected (E) ratios indicated a peak frequency of menarche in July (1.45) and January (1.19) and troughs in February (0.75) and May (0.79). Collapsing the data into seasons also revealed summer and winter peaks. Participants from a younger cohort (born after 1970) had an earlier age of menarche than participants from an older cohort (born before 1970). Older cohort participants also had a more pronounced December-January frequency peak and younger cohort participants had a more pronounced July frequency peak. Older cohort participants exhibited more variability in menarcheal age as a function of menarche month and birth month. Late-maturing participants (>14 years) had a larger December peak of menarche frequencies. Spring-born participants exhibited less seasonality to their menarche. Month of birth was not significantly associated with month of menarche; however, fewer cases than expected occurred 3 months after the birth month and more cases than expected occurred 6 months after the birth month. Age of menarche and monthly distribution of menarche did not vary according to latitude or altitude. Conclusions: Menarche seasonality is suggested to be a multifactor process mediated by the most prominent seasonal time cue. Attention to these cues that have been hypothesized to contribute to menarche seasonality (e.g., stress or the photoperiod) may have practical implications for women's health, given some of the risks associated with early menarche.