Abstract 873: Parity, breastfeeding and ovarian cancer in the Million Women Study
Authors: Gaitskell Kezia , Green Jane , Pirie Kirstin , Reeves Gillian , Beral Valerie 
Published: August 2015

Abstract
There is epidemiological evidence that parity is associated with a reduced risk of ovarian cancer, although few studies are large enough to adequately explore variation by histological subtype. Evidence of association with breastfeeding is varied, and often subject to significant residual confounding by parity.
We explored the association between ovarian cancer and childbearing patterns (parity and breastfeeding) in a large prospective cohort of UK women. Information on exposures was obtained from the baseline questionnaire. Using a Cox proportional hazards model, we estimated the relative risk (RR) of ovarian cancer in women with different childbearing patterns, after adjustment for potential confounding factors (including age, region, history of breast cancer, hysterectomy, tubal ligation, use of contraceptive or menopausal hormones, body mass index, smoking, and socioeconomic status). We used a competing risks approach to look at variation by histological subtype, and explored effects of the timing of births. Analyses of breastfeeding were restricted to parous women and stratified by parity.
After excluding women with previous cancer, bilateral oophorectomy, and unknown parity, the study population included 1,146,985 women, aged 56 years on average at recruitment. 7570 incident cases of ovarian cancer accrued, after an average of 13.0 years of follow-up. Overall, women with one child had an estimated 20% reduction in the relative risk of ovarian cancer compared to nulliparous women (RR: 0.80, 95%CI: 0.74-0.87). However, this showed significant heterogeneity by histological subtype (p-het = 0.005). There was strong evidence of a reduced risk of clear cell (n = 156, RR: 0.57, 95%CI: 0.41-0.79) and endometrioid tumours (n = 220, RR: 0.64, 95%CI: 0.49-0.84), but no statistically significant reduction of mucinous (n = 234, RR: 0.82, 95%CI: 0.63-1.06) or serous tumours (n = 916, RR: 0.92, 95%CI: 0.81-1.05). Amongst parous women, each additional birth was associated with an estimated 9% reduction in the relative risk of ovarian cancer (RR: 0.91, 95%CI: 0.88-0.94), without significant subtype heterogeneity (p-het = 0.1). We found no evidence of a significant association with the age at first or last birth.
After careful adjustment for parity, we found limited evidence for a reduction in ovarian cancer risk with breastfeeding. Overall, breastfeeding (ever vs never) was not associated with ovarian cancer (RR: 0.94, 95%CI: 0.89-1.00, p = 0.07), and no variation was seen by histological subtype (p-het = 0.7). However, there was a significant trend with duration of breastfeeding (p-trend &lt;0.001).
In this largest prospective study of ovarian cancer to date, our results provide new reliable evidence that childbearing has different effects on different subtypes of ovarian cancer.
Citation Format: Kezia Gaitskell, Jane Green, Kirstin Pirie, Gillian Reeves, Valerie Beral. Parity, breastfeeding and ovarian cancer in the Million Women Study. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 873. doi:10.1158/1538-7445.AM2015-873