Childbearing and the Risk of Bladder Cancer: A Nationwide Population-based Cohort Study

BackgroundThe incidence of bladder cancer (BCa) is substantially lower in women than in men, a difference that cannot be fully explained by established risk factors. Several studies suggest that hormonal and reproductive factors play a role in the development of BCa.ObjectiveTo examine possible associations between patterns of childbearing and the risk of BCa.Design, setting, and participantsThis cohort study encompassed >2 million women for whom information on reproductive history and BCa incidence was retrieved from Swedish population-based registers.Outcome measurements and statistical analysisIncidence rate ratios (IRRs) of BCa were estimated using Cox proportional hazards modelling. The exposures under investigation were parity and age at first birth, adjusted for education and history of chronic obstructive lung disease (COLD).Results and limitationsAmong 2 009 811 women in the cohort, 2860 incident cases of BCa were identified. Parous women had lower incidence of BCa compared with nulliparous women (adjusted IRR: 0.80; 95% confidence interval [CI], 0.72–0.89). Moreover, the incidence was 15% lower in women with two children and 24% lower in women with three or more children compared with uniparous women. Compared with women aged 20–24 at first childbirth, the incidence was elevated in women with a first birth before age 20 (adjusted IRR: 1.16; 95% CI, 1.05–1.29). The risk of BCa was elevated in women with low education and among women with a history of COLD. Absence of data on menstrual history, use of exogenous hormones, and smoking was a limitation of the study.ConclusionsThe incidence of BCa decreased with increasing parity and older age at first birth. Although smoking habits may partly explain some of the associations, our findings provide support for yet-to-be-identified protective mechanisms associated with childbearing, possibly mediated by hormonal or structural changes following pregnancy.