Multiple births and breast cancer prognosis: a population based study.

Survival in relation to endocrine or reproductive factors has rarely been studied since the focus of most studies has been on the risk of breast cancer. In this study we analysed the effect of multiple birth compared to single births on breast cancer survival. A population based cohort of 30619 women born after 1935 and diagnosed with primary breast cancer, between 1958 and 1998, was generated by linking a number of Swedish registries including Swedish Cancer Registry, Cause of Death Registry, Swedish Generation Registry and the Registry of Population and Population changes. Then we quantified the association between singleton and multiple births to breast cancer specific fatality using the Cox proportional hazards model. We found the singleton and multiple births were associated with increased fatality if breast cancer was diagnosed within 5 years of childbirth, this effect dying out to nil at 5 years after childbirth. Thereafter there was a protective effect on survival with time. Although childbirth is known to decrease breast cancer risk, this study demonstrates that a survival disadvantage exists for breast cancer diagnosed within the first 5 years of singleton and multiple births. It is concluded that breast cancer diagnosed within 5 years of childbirth, particularly if this or prior pregnancies have been multiple, should be regarded as a negative prognostic factor and considered in counselling and treatment of these patients.