Season of birth of breast cancer patients and its relation to patients' reproductive history in Tokyo, Japan.

Seasonal distribution of the birth dates of 405 pre-menopausal and 285 post-menopausal breast cancer patients was investigated in order to determine whether or not the season of their birth was related to various reproductive risk factors of breast cancer, including nulliparity, late age at first birth, early age at menarche, late age at menopause, and a history of benign breast diseases. The seasonal distributions of births were compared between groups of patients categorized according to whether they possessed each risk factor or not, separately for pre- and post-menopausal patients. Patients with the same menopausal status generally had the same seasonal distribution of births, irrespective of whether or not they possessed a risk factor. Moreover, low-risk patients exhibited more deviation in the seasonal distribution of birth from general births than the high-risk patients. These results suggest that the distinctive seasonal distribution of birth observed in breast cancer patients is basically a phenomenon independent from the effect of the reproductive history on the occurrence of breast cancer, and that specific seasonal factors are involved at the fetal or neonatal stage in the etiology of breast cancer.