Impact of parity on the incidence of ovarian cancer subtypes: a population-based case–control study

Background: Parity is known to have a protective effect as regards ovarian cancer, but its effect on the different histological subtypes of ovarian cancer is not well known. The impact of parity on the incidence of ovarian cancer subtypes was studied. Material and methods: All Finnish women diagnosed 1994–2013 with ovarian cancer for the first time were included. Altogether, 5412 cases of ovarian cancer were identified in the Finnish Cancer Registry and stratified according to morphology into serous, mucinous, endometrioid, clear cell and others. Five age-matched controls were randomly selected for each case from the Finnish National Population Registry. Data on postmenopausal hormonal therapy were derived from the Registry of Prescribed drugs and used as cofactors. Multivariate conditional logistic regression for matched case-control data was used to examine the associations between parity parameters and ovarian cancer risk. Results: Parous women had lower risk than nulliparous women in getting ovarian cancer of any type under age of 55 years. The odds ratio (OR) for serous cancer was 0.65 (95% confidence interval 0.56–0.77), for mucinous cancer 0.66 (0.52–0.83), for endometrioid cancer 0.52 (0.40–0.68), for clear-cell cancer 0.30 (0.19–0.46) and for other types 0.59 (0.43–0.80). In women aged 55 or older, the respective ORs were 0.86 (0.75–0.99), 0.78 (0.57–1.07), 0.61 (0.47–0.79), 0.44 (0.29–0.66) and 0.74 (0.57–0.95), adjusted for hormone therapy. Number of childbirths was associated with a trend toward reduction of risk, especially in serous and clear-cell cancers. Higher age at first birth was associated with higher risk of clear-cell cancer but otherwise age at first or last birth did not have an impact on the incidence of cancer subtypes. Conclusions: Childbirths decrease the risk of all histologic subtypes of epithelial ovarian cancer in women in premenopausal and postmenopausal age.