Gaitskell K, et al. - In the prospective Million Women Study, researchers focused on histological subtypes of ovarian cancer associated with parity and breastfeeding. As per findings, substantial variation regarding ovarian cancer risk associated with parity by tumour histotype was observed. Nulliparity seemed associated with a substantially greater overall risk than expected from the effect of a single birth, especially for clear cell and endometrioid tumours, perhaps suggesting an association of infertility with these histotypes.

• In a prospective study of 1.1 million UK women, 8719 were identified that developed ovarian cancer during follow-up.
• Amongst women with different childbearing patterns, Cox regression indicated adjusted relative risks (RRs) overall and by tumour histotype.
• 24% greater ovarian cancer risk was observed for nulliparous women in comparison to women with one child, with significant heterogeneity by histotype (p=0.01).
• Findings revealed no marked increase in serous tumours, a modest increase in mucinous tumours, but a substantial increase in endometrioid (RR=1.49, 95% CI: 1.18-1.89) and clear-cell tumours (RR=1.68, 1.29-2.20).
• Among parous women, each additional birth was associated with an ovarian cancer risk reduction by 6%; this association also varied by histotype (p=0.0006), with the largest reduction in risk for clear-cell tumours (RR per birth=0.75, 0.65-0.85, p<0.001) and weak, if any, effect for endometrioid, high-grade serous, or mucinous tumours.
• There appeared little association with age at first or last birth.
• There appeared about a 10% risk reduction per 12-months breastfeeding (RR=0.89, 0.84-0.94, p<0.001), with no marked heterogeneity by histotype, but statistical power was limited.