Seasonal variations in menarche in Oslo

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Abstract
Data from .apprx. 11,000 girls aged 10-18 yr were used to study seasonal variations in menarche in Oslo, Norway. A statistical method which takes into account the changes over time in the age-structure of the sample is used to show that throughout the period 1965-1970, the menarche incidence varied according to a stable bimodal seasonal pattern with peaks in Dec.-Jan. and July-Aug. This pattern corresponds to those observed in Sweden and Finland, but deviates from other reported patterns, i.e., from the variations found in Copenhagen [Denmark]. A possible cause of the general lack of well supported hypotheses for seasonal variations is that an environmental factor may cause marked cyclic variations, without having a marked effect on the process determining maturation.