ABSTRACT
Background: It has been hypothesized that the yearly menarche rhythm could be caused by the seasonal variation of photoperiod and temperature or by the annual distribution of the scholar vacation and study periods. Aim: To test the hypothesis that the distribution of study vacation periods is a condition that modifies the annual menarche rhythm. Subjects and methods: Two thousand ninety four school girls from Chile, 2.356 girls from Madras, India, 3.454 girls from Medellin, Colombia and 2.627 girls from Debrecen, Hungary, were studied. They were asked about the month of their menarche. Vacation months were considered those with more than 6 days of leave from school. Results: The seasonal hypothesis was refuted because there were contradictions with the expected antithetical behavior in both hemispheres, there was a significant heterogeneity of the yearly menarche among girls from the same region, the expected cline of the menarche frequency variance from equator to poles was not observed, finding an antithetical cline instead and there was a significant heterogeneity among months of the same season. On the other hand, months with vacation periods coincided significantly with peaks of menarche, while study months had lower proportion of menarche (total binomial probability <10-6). Girls whose month of menarche was the same as their month of birth, did not agree completely with the vacation-study hypothesis as the rest of the sample. Conclusions: Vacation periods influenced menarche rhythm. However, these periods coincide with most cultural events and this strong association needs further study to be considered causal. It is not possible to assume school stress as the main explanatory variable. Ontogenetic factors such as birth imprinting also can influence the menarche rhythm, as shown in girls whose month of menarche coincided with their month of birth