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Recent Childbirth a Risk Factor for Breast Cancer

Ann Intern Med; ePub 2018 Dec 11; Nichols, et al

Parous women have an increased risk for breast cancer for > 20 years after childbirth compared with nulliparous women, a recent study found. A pooled analysis of 15 prospective studies included women aged <55 years from the International Premenopausal Breast Cancer Collaborative Group. Researchers found:

  • During 9.6 million person-years of follow-up, 18,826 incident cases of breast cancer were diagnosed.
  • Compared with nulliparous women, parous women had a hazard ratio (HR) for breast cancer that peaked about 5 years (HR, 1.80) after birth before decreasing to 0.77 after 34 years.
  • Increases in breast cancer risk after childbirth were pronounced when combined with a family history of breast cancer.
  • Women who were older at first birth or who had more births were also at increased risk.
  • Breastfeeding did not modify overall risk patterns.


Nichols HB, Schoemaker MJ, Cai J, et al. Breast cancer risk after recent childbirth: A pooled analysis of 15 prospective studies. [Published online ahead of print December 11, 2018]. Ann Intern Med. doi:10.7326/M18-1323.