• Hazel B Nichols, Minouk J Schoemaker, Jianwen Cai, Jiawei Xu, Lauren B Wright, Mark N Brook, Michael E Jones, Hans-Olov Adami, Laura Baglietto, Kimberly A Bertrand, William J Blot, Marie-Christine Boutron-Ruault, Miren Dorronsoro, Laure Dossus, A Heather Eliassen, Graham G Giles, Inger T Gram, Susan E Hankinson, Judy Hoffman-Bolton, Rudolf Kaaks, Timothy J Key, Cari M Kitahara, Susanna C Larsson, Martha Linet, Melissa A Merritt, Roger L Milne, Valeria Pala, Julie R Palmer, Petra H Peeters, Elio Riboli, Malin Sund, Rulla M Tamimi, Anne Tjønneland, Antonia Trichopoulou, Giske Ursin, Lars Vatten, Kala Visvanathan, Elisabete Weiderpass, Alicja Wolk, Wei Zheng, Clarice R Weinberg, Anthony J Swerdlow, and Dale P Sandler.
• University of North Carolina Gillings School of Global Public Health, Chapel Hill, North Carolina (H.B.N., J.C., J.X.).
• Ann. Intern. Med. 2019 Jan 1; 170 (1): 223022-30.

BackgroundParity is widely recognized as protective for breast cancer, but breast cancer risk may be increased shortly after childbirth. Whether this risk varies with breastfeeding, family history of breast cancer, or specific tumor subtype has rarely been evaluated.ObjectiveTo characterize breast cancer risk in relation to recent childbirth.DesignPooled analysis of individual-level data from 15 prospective cohort studies.SettingThe international Premenopausal Breast Cancer Collaborative Group.ParticipantsWomen younger than 55 years.MeasurementsDuring 9.6 million person-years of follow-up, 18 826 incident cases of breast cancer were diagnosed. Hazard ratios (HRs) and 95% CIs for breast cancer were calculated using Cox proportional hazards regression.ResultsCompared with nulliparous women, parous women had an HR for breast cancer that peaked about 5 years after birth (HR, 1.80 [95% CI, 1.63 to 1.99]) before decreasing to 0.77 (CI, 0.67 to 0.88) after 34 years. The association crossed over from positive to negative about 24 years after birth. The overall pattern was driven by estrogen receptor (ER)-positive breast cancer; no crossover was seen for ER-negative cancer. Increases in breast cancer risk after childbirth were pronounced when combined with a family history of breast cancer and were greater for women who were older at first birth or who had more births. Breastfeeding did not modify overall risk patterns.LimitationsBreast cancer diagnoses during pregnancy were not uniformly distinguishable from early postpartum diagnoses. Data on human epidermal growth factor receptor 2 (HER2) oncogene overexpression were limited.ConclusionCompared with nulliparous women, parous women have an increased risk for breast cancer for more than 20 years after childbirth. Health care providers should consider recent childbirth a risk factor for breast cancer in young women.Primary Funding SourceThe Avon Foundation, the National Institute of Environmental Health Sciences, Breast Cancer Now and the UK National Health Service, and the Institute of Cancer Research.

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