Birth seasonality of breast cancer patients and its variation according to menopausal status and histologic type in Japan.

The season of birth of 828 Japanese female patients with breast cancer was compared with that of a general control population. A distinct seasonal deviation was detected in the birth of breast cancer patients. In general, a major peak of birth occurred during spring to autumn, and a trough appeared in winter. However, the birth seasonal distributions of pre- and post-menopausal patients groups were not identical. The pre-menopausal patients had a summer birth excess and a clear sinusoidal distribution. The post-menopausal patients showed two peaks in spring and autumn. Such differences according to menopausal status were consistently observed when patients were divided by birth-year period and by histological subtype. These findings suggest the possibility of involvement of some exogenous seasonal factors acting at the fetal or neonatal stages in the etiology of breast cancer, and that the characteristics of the development of breast cancer differ in pre-menopausal women in comparison with those in post-menopausal women.