Parity, reproductive factors, and the risk of pancreatic cancer in women.

Incidence rates for pancreatic cancer are consistently lower in women than in men. Previous studies suggest that reproductive factors, particularly parity, may reduce pancreatic cancer risk in women. We examined parity, breast feeding history, age at first birth, menstrual factors, and exogenous hormone use in relation to pancreatic cancer risk in a prospective cohort study of women. Information on parity and other reproductive factors was assessed by questionnaires in 1976 and updated biennially. Multivariate relative risks were adjusted for cigarette smoking, body mass index, diabetes, and height. During 22 years of follow-up (1976-1998), 115,474 women contributed 2.4 million years of person time, and 243 cases of pancreatic cancer were identified. Compared with nulliparous women, the relative risk of pancreatic cancer was 0.86 [95% confidence interval (CI), 0.55-1.36] for women with 1-2 births, 0.75 (95% CI, 0.48-1.17) for 3-4 births, and 0.58 (95% CI, 0.34-0.98) for those with >/=5 births after adjusting for other factors. An analysis for linear trend indicates a 10% reduction in risk for each birth (P(trend) = 0.008). Other reproductive factors and exogenous hormone use were not significantly related to pancreatic cancer risk. In this large prospective cohort of women, parity was associated significantly with a reduced risk of pancreatic cancer. Additional studies should examine the physiological or hormonal changes underlying pregnancy or childbirth that may explain this finding.