The association of reproductive factors and breastfeeding with long term survival from breast cancer.

Reproductive factors that influence breast cancer risk may also have an impact on survival, once the disease is diagnosed. In this study, 2,640 women were diagnosed with invasive breast cancer during follow-up after a breast cancer screening that took place in 1956-1959. Survival was assessed in relation to age at menarche, age at first birth, parity, history of breastfeeding, age at menopause, and the effect of BMI was assessed in a subset of patients. It is a special feature that the patients of this study have not been subjected to organized mammography screening and their use of exogenous hormones has been negligible. By the end of follow-up (2008), 2,301 (87%) of the patients had died and 1,022 (44%) of the deaths were caused by breast cancer. Breast cancer survival was not associated with age at menarche, parity or time since last birth, but survival was consistently poorer with increasing age at first birth (P for trend 0.03): comparing a first birth after 35 years with 25-29 years, the hazard ratio was 1.32 (95% CI 1.02-1.72). There was no evidence for a dose-related effect of breastfeeding, but BMI measured many years prior to diagnosis was inversely associated with survival (P for trend <0.01). The main finding was that reproductive factors, including breastfeeding, appear to have little influence on the survival of breast cancer patients. Age at first birth may be an exception to this, since we found a gradually poorer survival with increasing age at first birth. We also found that overweight and obesity, as measured many years prior to diagnosis, were associated with poorer survival.