Seasonal variations in menarche in Oslo.

Data from about 11,000 girls aged 10-18 years 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 December-January and July-August. This pattern corresponds to those observed in Sweden and Finland, but deviates from other reported patterns, i.e. from the variations found in Copenhagen. It is argued that a possible cause of 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.