1. The authors of the study concluded that parous women may have an elevated breast cancer risk for over 20 years following delivery.
2. The increased risk for breast cancer in parous women compared to nulliparous women was greatest at 5 years postpartum.
Evidence Rating Level: 1 (Excellent)
Study Rundown: Breast cancer is an increasingly common malignancy found in our population. One widely recognized protective factor for breast cancer is childbirth, where women who have children at an older age or who are nulliparous are thought to have a higher risk of developing breast cancer later in life. While parity has been shown to be protective later in life, the authors of this study aimed to evaluate whether women had increased protection from breast cancer shortly after delivery. Pooled analysis showed that breast cancer risk was increased in parous women compared to nulliparous women for over 20 years with a peak risk at 5 years postpartum. Strengths of the study included the large number of cases in the pooled analysis of prospective cohorts. One of the main limitations in this study was the identification of timing of breast cancer diagnosis. The authors acknowledged that breast cancer diagnoses during pregnancy were not always distinguishable from early postpartum diagnoses.
Relevant Reading: Pregnancy-related Characteristics and Breast Cancer Risk
In-Depth [prospective cohort]: The authors of this study conducted a pooled analysis of individual-level data from 15 prospective cohorts which were a part of the Premenopausal Breast Cancer Collaborative Group. The authors aimed to assess the risk of breast cancer related to recent childbirth. Participants were restricted to women younger than 55 years who did not have breast cancer prior to commencement of the study. The authors calculated hazard ratios comparing parous versus nulliparous women, looking at a total of 9.6 million person-years of follow-up. During this time frame, a total of 18 826 incident cases of breast cancer were diagnosed. When comparing parous to nulliparous women, parous women had a HR for breast cancer associated with time since recent birth that was highest after 4.6 years postpartum (HR, 1.80 [95% CI, 1.63 to 1.99]). The HR decreased to 0.77 34.5 years after birth (CI, 0.67 to 0.88) with a change to decreased risk occurring at 23.6 (95% CI, 21.9 to 25.0) years after childbirth primarily due to a change in the incidence of estrogen receptor-positive breast cancer.
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