Breast Cancer Risk After Recent Childbirth : A Pooled Analysis of 15 Prospective Studies

Background: Parity 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. Objective: To characterize breast cancer risk in relation to recent childbirth. Design: Pooled analysis of individual-level data from 15 prospective cohort studies. Setting: The international Premenopausal Breast Cancer Collaborative Group. Participants: Women younger than 55 years. Measurements: During 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. Results: Compared 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. Breast cancer risk increases after childbirth were pronounced when combined with a family history of breast cancer, and greater for women who were older at first birth or who had more births. Breastfeeding did not modify risk patterns. Limitations: Breast 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. Conclusion: Compared 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. Abstract Parity is widely recognized as protective for breast cancer, but there is evidence that breast cancer risk may increase shortly after childbirth. Whether this risk varies with such factors as breastfeeding, family history of breast cancer, or specific tumor subtype has rarely been evaluated. To characterize breast cancer risk in relation to recent childbirth, this study pooled individual-level data for women younger than 55 years from 15 prospective cohort studies. Breast cancer is the leading cancer diagnosis among reproductive-aged women worldwide (1). Parity is recognized as a protective factor for breast cancer overall, but this may largely apply to the peak ages of incidence (after age 60 years) and may not be true for younger women. Previous studies have reported that recent childbirth confers a short-term increase in breast cancer risk (2–13), which may last 10 or more years (6, 11, 14–16) and be amplified in women who are older at first birth (6, 11, 15, 16). Evidence for this increased risk often comes from national registry linkage studies in Scandinavian countries (2, 4, 6, 17). Information about such behaviors as breastfeeding is often not available or comes from Nichols et al. Page 5 Ann Intern Med. Author manuscript; available in PMC 2019 September 25. A uhor M anscript