ANNUAL RHYTHM OF MENARCHE IN MEDELLIN, CO LOMBIA

Hypotheses on seasonal, phylogenetic and ontogenetic factors or imprin ting that may produce menarcheal rhythm were tested in a sample of sch ool girls from Medellin, Colombia. The questionnaire included a net of important religious or national feasts and periods of vacation or stu dy to prevent memory biases. European, Asian and Chilean samples showe d peaks of menarche in December and January, but a few samples showed a second one in June, July and August. The winter short photoperiod an d low temperature were assumed to be the cause of the winter peak in m ost European samples. However, this seasonal hypothesis was refuted wh en a Chilean sample also showed a peak in December, January and Februa ry (Summer). In the present study performed in a sample from a tropica l country we found two peaks of menarche, one in July and the other in November, December and January; thus, the seasonal hypothesis can be hardly supported. Moreover, this sample showed a high coincidence betw een the month of menarche and the month of birth, and a different patt ern of menarche when menarches are distributed according to their coin cidence with the gestational development. Both facts affirm the ontoge netic hypothesis, because they were also found in several samples. Sin ce this population is different from those already studied and its mon thly distribution of menarches and births is also different from those other ones, the hypothetical phylogentic factors find additional supp ort. The possible influence of school stress and other environmental f actors in the menarcheal rhythm are discussed.