Parity and breast cancer.

Data from the 1961 and 1971 Censuses in England and Wales were used to estimate the age distribution of women of various parities in 1976. Applying the age-specific incidences of breast cancer for women in England and Wales in 1975 gave the expected number of cases of that disease in 1976 and permitted an estimate of the mean age at diagnosis of breast cancer at each parity. This showed that the highest average age for breast cancer occurred in nulliparous women (65.9 years) and that the lowest age for the disease occurred in women who had borne two children (60.4 years). The figures obtained were similar to those reported in a separate study of women treated in Birmingham. The results of that study, however, may have been due to the age distribution in the population of women by parity, rather than any direct influence of parity on the speed of growth of breast cancer.