Women who are older at their first birth, multiparous, or who have a family history of breast cancer are at increased risk for breast cancer, according to a study published in Annals of Internal Medicine.

An international team of researchers conducted a pooled analysis of the international Premenopausal Breast Cancer Collaborative Group to distinguish the correlation between breast cancer risk and recent childbirth in women younger than 55 years.

A total of 18,826 incident cases of breast cancer were diagnosed in 9.6 million person-years of follow-up. The hazard ratio for breast cancer among parous women compared with nulliparous women was 1.80 at nearly 5 years after birth and dropped to 0.77 after 34 years; the link between cancer and years since childbirth shifted from positive correlation to negative at approximately 24 years.

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The investigators noted that the association was influenced by estrogen receptor-positive breast cancer and that no switch in correlation was observed for estrogen receptor-negative breast cancer.

When coupled with a family history of breast cancer, risk after childbirth was evident and more pronounced for women who were older at their first birth or who had multiple births; breastfeeding did not affect the overall risk patterns.

One limitation the investigators mentioned was the lack of data on human epidermal growth factor receptor 2 oncogene overexpression.

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“Compared with nulliparous women, parous women have an increased risk for breast cancer for more than 20 years after childbirth,” the authors wrote. “Healthcare providers should consider recent childbirth a risk factor for breast cancer in young women.”


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