[Menstruation pattern and month of birth].

By using the Nijmegen data base on breast cancer we attempted to determine whether a relationship exists between menstrual pattern and month of birth and if so, if its agreed with the hypothesis of Seasonal Preovulatory Overripening Ovopathy (SPrOO). On the basis of the birth curve of women who participated in the Nijmegen breast cancer screening programme (1975) risk months for SPrOO and reference months were defined. We found that more women born in a risk month had an early menarche (< or = 11 years), a long menstrual cycle (> or = 5 weeks; QI: 20-25 kg/m2), menorrhagia, and an early (35-40 years) or late (> or = 56 years) menopause age. This relationship was in line with the predictions according to the SPrOO hypothesis. In all, a positive (but not always significant) relationship appeared to exist in 12 of the 15 associations studied. We conclude that there is a relationship between menstrual pattern and month of birth, which can be explained by the SPrOO hypothesis.