Increasing parity and older age at first birth are associated with a decreased risk for developing bladder cancer, according to researchers.

Caroline E. Weibull, MSc, of the Karolinska Institutet in Stockholm, Sweden, and colleagues studied a cohort of more than two million Swedish women in which 2,860 new cases of bladder cancer were identified. Women who had children had a lower incidence of bladder cancer than women who did not. In addition, the incidence of bladder cancer was 15% lower in women with two children and 24% lower in women with three or more children compared with women who had one child, Weibull’s group reported online in European Urology.

The incidence of bladder cancer was lower among women who had their first child when they were 20-24 years old than women who had their first children before age 20.

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The risk of bladder cancer was higher in women with low educational attainment and those with a history of chronic obstructive lung disease.

The study was limited by an absence of data on menstrual history, use of exogenous hormones, and smoking, the researchers noted.

“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,” the authors concluded.