Parity, age at first birth, breastfeeding and breast cancer risk: a nationwide prospective study of 300,000 Chinese women

Ling YANG, University of Oxford, United Kingdom
LI L. 2,3 , CHEN Y. 1 , GUO Y. 2 , PETO R. 1 , CHEN Z. 1

1 1 CTSU, Nuffield Department of Population Health, University of Oxford, UK; 2 Chinese Academy of Medical Sciences, Dong Cheng District, Beijing, China; 3 Department of Public Health, Beijing University, Beijing, China

Background: Previous studies, mostly from West have reported inconsistent findings on the associations between childbearing related aspects and risk of breast cancer. Little is known in China where women’s reproductive factors patterns differ importantly from the West and the incidence of breast cancer rapidly increased.
Methods: The China Kadoorie Biobank recruited 302,632 women aged 30-79 (mean (SD) 50.8 (10.6)) years in 2004-8 from 10 diverse regional sites across China, including 55.6% postmenopausal women. During 7 years follow-up, 1,353 incident breast cancer cases were reported among 284970 women who had no prior history of cancer at baseline. Cox regression yielded adjusted hazard ratios (HRs) to breast cancer risk.
Results: Most women were parous (98.7%) with means of number of livebirths 2.3 (1.3), age at firth birth at 23.4 (3.2) years and total breastfeeding 34.8 (24.7) months. After adjustment of various confounders, parity significantly reduced breast cancer risk, with a 19% (11-25%) reduced risk for every one more childbearing (adjusted HRs of 1.00 (0.85, 1.17), 0.85 (0.78, 0.92), 0.65 (0.55, 0.78) and 0.54 (0.42, 0.69) for women with one (reference), two, three and four or more livebirths, respectively). No material change on the association was found in subgroups defined by area and women’s menopausal status. A slightly J-shaped association was observed between age at first birth and breast cancer risk. Parous women who ever breastfed her children had 25% (3-41%) lower risk of breast cancer than those who never done so. The adjusted HRs of breast cancer were 1.20 (0.94, 1.54), 1.00 (0.86, 1.16), 0.99 (0.89, 1.11), 0.90 (0.78, 1.03), 0.87 (0.71, 1.06), 0.88 (0.70, 1.11) for women never breastfed, total breastfed duration of <1, 1, 2, 3 and ≥4 years, respectively. 
Conclusion: The more children women having or the longer women breastfeed, the more they are protected against breast cancer.