Seasonal characteristics of and age at menarche.

Data from approximately 600 U.S. women were analyzed for seasonality in menarche. Data were converted to the ratio of observed (O) to expected (E) cases, based on an equal distribution of events over the year, per considered time span of the year. The greatest O/E ratio in menarche (1.55) occurred during January, followed by August (1.43) and July (1.24). The lowest O/E ratio in menarche occurred in February and May (0.68). Seasonal patterns in menarche were detected for the separate cohorts of women born during the 1940s, 1950s, and 1960s, with a shift from a December-January peak in menarche for those born in the 1940s and 1950s to an August-September peak for those born in the 1960s. Girls who were younger (8-14 yrs) at menarche exhibited a seasonal difference in the peak number of menarches by about 6 months in comparison to the pattern for girls who were older (15-17 yrs). Girls who experienced menarche during August-October were statistically significantly younger (p less than 0.05) than those born during the other three seasons. Season of birth was not statistically significantly associated with season of menarche. Overall, no 12-month pattern was substantiated by cosinor analysis in the month of menarche. However, a 6-month rhythm was detected in menarche for women born between 1940 and 1960 (p = 0.004, A = 29% M, phi = January and July).