Beyond parity: association of ovarian cancer with length of gestation and offspring characteristics.

Women with higher parity have a lower risk of ovarian cancer possibly because of pregnancy hormones, but the specific effect of different pregnancy hormones on ovarian cancer risk is not clear. Some clarification might be gained by considering situations where hormone levels vary between pregnancies. Study participants from an Australian population-based, case-control study of epithelial ovarian cancer (2001-2005) completed a reproductive/lifestyle questionnaire. The authors included 1,203 cases and 1,286 controls with at least 1 birth and, using multiple logistic regression, calculated odds ratios and 95% confidence intervals to investigate the effects of pregnancy-related factors on cancer risk. Women who had 1 or more preterm births had higher risks of ovarian cancer than those who had only full-term births (odds ratio (OR) = 1.48, 95% confidence interval (CI): 1.02, 2.15). The authors also found that bearing only boys was associated with a 2-fold increased risk of mucinous ovarian cancer (OR = 2.19, 95% CI: 1.15, 4.17). There was no association between multiple pregnancy and ovarian cancer (for any multiple pregnancy vs. only singleton pregnancies: OR = 1.22, 95% CI: 0.74, 2.02). The results suggest that pregnancies associated with differing hormonal milieux have different effects on ovarian cancer risk and that some of these associations may vary with histologic subtype.