Month at menarche: a re-evaluation of the seasonal hypothesis.

The hypothesis of a dominant or major seasonal factor that produces the winter peak of menarche found in European girls was tested using several Caucasian and Mongolian samples from the northern hemisphere and two Chilean samples. Present results do not support such a hypothesis. The month at menarche of Chilean girls did not show the expected winter peak; February, a winter month in Europe, showed a deep trough in most samples. Instead, several Mongolian and Caucasian samples from the northern hemisphere clustered according to their ethnic origin rather than to the expected northern hemisphere yearly distribution of photoperiods and temperatures. Other non-seasonal factors seem to explain much better the periodicity of the monthly incidence of menarche. A phylogenetic factor (imprinting) is proposed to explain peaks seen in December and January for Caucasian, Mongolian, Finnish and Chilean samples living in the northern and southern hemispheres. Ontogenetic factors are proposed to explain the highly significant coincidence between the month of menarche and the month of birth, and the modulation of the phylogenetic imprinting by months at which gestation occurred in relation to menarche. The Finnish and Chilean samples, which can be considered mixed Caucasian and Mongolian, showed an intermediate pattern between the Caucasian and Mongolian groups.