[Annual rhythm of menarche in Medellín, Colombia]

Hypotheses on seasonal, phylogenetic and ontogenetic factors or imprinting that may produce menarcheal rhythm were tested in a sample of school girls from Medellín, Colombia. The questionnaire included a net of important religious or national feasts and periods of vacation or study to prevent memory biases. European, Asian and Chilean samples showed peaks of menarche in December and January, but a few samples showed a second one in June, July and August. The winter short photoperiod and low temperature were assumed to be the cause of the winter peak in most European samples. However, this seasonal hypothesis was refuted when a Chilean sample also showed a peak in December, January and February (Summer). In the present study performed in a sample from a tropical country we found two peaks of menarche, one in July and the other in November, December and January; thus, the seasonal bypothesis can be hardly supported. Moreover, this sample showed a high coincidence between the month of menarche and the month of birth, and a different pattern of menarche when menarches are distributed according to their coincidence with the gestational development. Both facts affirm the ontogenetic hypothesis, because they were also found in several samples. Since this population is different from those already studied and its monthly distribution of menarches and births is also different from those other ones, the hypothetical phylogenetic factors find additional support. The possible influence of school stress and other environmental factors in the menarcheal rhythm are discussed.