A prospective study of the independent effects of parity and age at first birth on breast cancer incidence in England and Wales.

The independent effects of parity and age at first birth on breast cancer incidence are investigated in a 1% sample of women aged 16 to 59 from the 1971 Census of England and Wales. Over the period 1971-81, 1,003 breast cancer cases occurred in the cohort of 113,263 women who were either married, widowed or divorced at the time of Census. Age at first birth was positively related to breast cancer risk, women giving birth to their first child after 35 years being at greater risk than nulliparous women. This effect remained, after adjustment for number of live-born children. Breast cancer risk showed a statistically significant decline with increasing parity even after adjustment for age at first birth. These results are consistent with other published evidence which suggests that other births subsequent to the first have an independent effect on breast cancer risk.