Secular trends in the distributions of the breast cancer risk factors--menarche, first birth, menopause, and weight--in Hiroshima and Nagasaki, Japan.

The results of a mail survey completed by approximately 21,000 female atomic bomb survivors in Hiroshima and Nagasaki in 1970 are studied with respect to menarche, first birth, menopause, and weight. These known risk factors for breast cancer can be used to explain some but not all of the differences in Japanese and US breast cancer rates. The results for age at menarche are the most striking, with the presence of a strong secular trend. The average age at menarche in Japan was observed to decrease from 16.4 years for women born in 1902 to 14.4 years for women born in 1942. However, a temporary increase was observed in women whose menarche was expected to occur during the war years of the early 1940s. Differences between US and Japanese women were also observed in age at first birth and nulliparity rates. Age at menopause was similar for the two groups, although US women have a considerably higher rate of surgically induced menopause. Within each birth cohort, it is shown that body weight is negatively correlated with age at menarche and positively correlated with age at menopause.