Monthly distribution of menarche in three provinces of north Italy.

Data on recalled month of menarche and month of birth have been collected in a sample of 1505 secondary-school students resident in three provinces of North Italy, to examine the monthly distribution of menarche and the relationship between season of birth, season of menarche and age at menarche. Menarche occurrence showed peaks of frequency in January and July-September, and troughs in October-November and February-May. This pattern was consistent with those reported in other Italian areas. Cases of coincidence between month of menarche and month of birth were significantly more frequent than expected at random. Mean age at menarche varied significantly according to the season at which menarche occurred, and a different pattern in the monthly distribution of menarche has been shown in early- and late-maturing girls. The distribution pattern of menarche occurrence observed in North Italy appears to be related to the rhythm of schoolwork activity. The possible influence of psychosocial stress on the trends in evidence is discussed.