Worse survival in breast cancer among women with recent childbirth: results from a Swedish population-based register study.

This study was designed to investigate how time since childbirth affects breast cancer survival using unselected population-based data linking data from the Swedish Cancer Registry, fertility register, and population census registers. A total of 14,693 parous women < or =45 years of age with breast cancer were identified. Information on deaths was collected, and 5- and 10-year survival rates were calculated according to time since most recent childbirth. Mortality during the first 10 years of follow-up was further investigated in a Cox analysis, with adjustments for age at diagnosis, time period during which the diagnosis was made, number of children, and age at the time of the first child's birth. Women with diagnosis during pregnancy had a 5-year survival rate of 52.1% (95% CI, 41.2%-61.9%) and a 10-year survival rate of 43.9% (95% CI, 33.1%-54.2%), compared with survival rates of 80.0% (95% CI, 79.6%-81.4%) and 68.6% (95% CI, 67.5%-69.7%), respectively, in women diagnosed >10 years since childbirth. In the multivariate model, we found that time since childbirth was associated with inferior survival rates in cases of diagnosis <8 years after childbirth, in which the lowest survival rates were seen in women with diagnosis during pregnancy in the first 5 years of follow-up (adjusted relative risk compared with women with >10 years since last childbirth, 2.6; 95% CI, 1.8-3.4). The adjusted hazard ratios could be described by a decreasing function of a logarithmic transformation of years since childbirth. We found that the time of follow-up was of importance, in that women with a recent pregnancy had particularly lower survival rates during the first 5 years after diagnosis. The mechanisms behind the poor survival in breast cancer for women with recent childbirth are not known, but we suggest that one explanation might be a lower proportion of hormone receptor-positive tumors.