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.