LAUSR

Epithelial ovarian cancer is a fatal disease of largely unknown etiology. Higher parity is associated with reduced risk of ovarian cancer. However, among parous women, the impact of pregnancy‐related factors on risk is not well understood. This population‐based case–control study included all parous women with epithelial ovarian cancer in Denmark, Finland, Norway and Sweden during 1967–2013 (n = 10,957) and up to 10 matched controls (n = 107,864). We used conditional logistic regression to estimate odds ratios (ORs) with 95% confidence intervals (CIs) for pregnancy‐related factors and ovarian cancer risk by histological subtype. Preterm delivery was associated with an increased risk [pregnancy length (last pregnancy) ≤30 vs. 39–41 weeks, OR 1.33 (95% CI 1.06–1.67), adjusted for number of births]; the OR increased as pregnancy length decreased (p for trend < 0.001). Older age at first and last birth was associated with a decreased risk [first birth: 30–39 vs. <25 years: adjusted OR 0.76 (95% CI 0.70–0.83); last birth 30–39 vs. <25 years: adjusted OR 0.76 (95% CI 0.71–0.82)]. Increasing number of births was protective [≥4 births vs. 1; OR 0.63 (95% CI 0.59–0.68)] for all subtypes, most pronounced for clear‐cell tumors [OR 0.30, (95% CI 0.21–0.44), pheterogeneity < 0.001]. No associations were observed for multiple pregnancies, preeclampsia or offspring size. In conclusion, in addition to high parity, full‐term pregnancies and pregnancies at older ages were associated with decreased risk of ovarian cancer. Our findings favor the cell clearance hypothesis, i.e. a recent pregnancy provides protection by clearing of precancerous cells from the epithelium of the ovary/fallopian tubes, mediated by placental or ovarian hormones.