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Increased Risk for Breast Cancer After Childbirth May Last More Than 2 Decades

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Key Points

  • Compared with women of the same age who had never given birth, women who had given birth had an increased risk for developing breast cancer that peaked about 5 years after childbirth and lasted about 20 years.
  • The increased risk was for both ER-positive and ER-negative breast cancers and was not mitigated by breastfeeding.

Although parity—time since most recent birth— is recognized as a protective factor against breast cancer, an analysis of data from 15 prospective cohort studies to assess breast cancer risk in relation to recent childbirth has found that compared to women of the same age who had never given birth, women who had given birth had an increased risk of developing breast cancer. That risk peaked about 5 years after childbirth and lasted about 20 years. Moreover, the increased risk was present for both estrogen receptor–positive and estrogen receptor–negative breast cancers and was not mitigated by breastfeeding.

The study’s findings should be considered in combination with other breast cancer risk factors (including older age at first birth, number of children, and family history of breast cancer) and be taken into account when assessing individual risk profiles for breast cancer. The study by Nichols et al was published in Annals of Internal Medicine.

Study Methodology

The researchers analyzed data from 889,944 women enrolled in 15 prospective cohort studies in the Premenopausal Breast Cancer Collaborative Group, a pooling project of studies participating in the National Cancer Institute Cohort Consortium. The women were younger than age 55 and did not have breast cancer at the time of enrollment.

The women were asked about their demographic characteristics, lifestyle, reproductive and breastfeeding history, medical conditions, and first-degree family history of breast cancer at enrollment and during each round of follow-up. Parity, breastfeeding, and family history of breast cancer were analyzed as time-varying exposures over follow-up.

During 9.6 million person-years of follow-up, 18,826 incident cases of breast cancer were diagnosed. Cox proportional hazards regression was used to calculate hazard ratios (HRs) and confidence intervals (CIs) for the association between time since most recent birth and breast cancer, with attained age as the underlying time scale.

Study Results

The researchers found that compared with nulliparous women, parous women had a hazard ratio for breast cancer that peaked about 5 years after giving birth (HR = 1.80, 95% CI = 1.63–1.99) before decreasing to 0.77 (95% CI = 0.67–0.88) after 34 years. The association crossed over from positive to negative about 24 years after birth.

The overall pattern was driven by estrogen receptor–positive breast cancer; no crossover was seen for estrogen receptor–negative cancer. Increases in breast cancer risk after childbirth were pronounced when combined with a family history of breast cancer and were greater for women who were older at first birth or who had more births. Breastfeeding did not modify overall risk patterns.

“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,” concluded the study authors.

Hazel B. Nichols, PhD, Assistant Professor in the Department of Epidemiology at the University of North Carolina Gillings School of Global Public Health, is the corresponding author of this study.

Disclosure: The study authors’ full disclosures can be found at annals.org.

The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.


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