We studied the monthly distribution of menarche among schoolgirls from Barrinha, Brazil, a municipality of rural characteristics in the process of transition to urbanization, economically dependent on sugar cane culture and processing. The association between season of the year, month of the year, and birth month and the occurrence of menarche was evaluated. The schoolgirls who provided the date of menarche (month and year) by the recall method and who had menarche up to 36 months before the interview were selected for analysis (460 girls). The correlations between month of occurrence of menarche and month of birth and between menarche and seasons of the year were determined by the chi-square test and the mean monthly ages at menarche were compared by Student's t-test, with a significance level of 0.05. There was a correlation between month of birth and month at menarche, with a peak of occurrence of menarche in December not affected by the menstruating patterns of maturation, but no correlation between menarche and seasons of the year. The difference between mean age at menarche in December (spring-summer transition) and in June (autumn-winter transition) was not significant. Mean age at menarche during the month following school vacations was lower. We conclude that situations of stress-relaxation transition, such as end of the school year, beginning of school vacation, and, perhaps most important, the socioeconomic factor-the end of the sugar cane harvest, when rural workers receive their final payments-favor the occurrence of the peak of menarche in December.
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