The seasonal rhythm of menarche in girls attending schools in Baghdad

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Abstract
1. The seasons and years of first menstrual period of 5000 girls aged 13-18 years, attending schools in Baghdad, were investigated retrospectively. 2. There was a highly significant seasonal rhythm of menarche with the peak in summer (July) and lowest extreme in winter (December) and autumn (November). This rhythm was the same, irrespective of whether girls had their first menses recently or not, and whether they were early or late maturers. 3. No significant association was found between the month of birth and the month of menarche. 4. No significant influence of age at menarche on its seasonal distribution was found. 5. There was a close, though not exact, relation between the seasonal change in day length and the rhythm of menarche. 6. The incidence of conception showed a significant seasonal rhythm but not coincident with menarche; the lowest level was in spring rather than in autumn or winter, when the day length is shortest.

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