MedicalResearch.com Interview with:
Dale P. Sandler, Ph.D.
Chief, Epidemiology Branch
National Institute of Environmental Health Sciences
MedicalResearch.com: What is the background for this study?
Response: Not having children is a well-established risk factor for breast cancer, but most of this evidence comes from studies of postmenopausal women since breast cancer before menopause is relatively uncommon. There is growing evidence that some risk factors differ for premenopausal and postmenopausal breast cancer – for example obesity which increases risk for breast cancer after menopause but appears to be protective before menopause.
There was some evidence that breast cancer risk increased shortly after pregnancy. It was thought that this risk lasted for 5 to ten years. Studies were unable to fully characterize the duration of this increase in risk or evaluate factors such as breast feeding, age at birth, or family history of breast cancer that could modify the relationship between recent pregnancy and breast cancer risk. Breast cancer before menopause or age 55 is relatively rare, and few individual studies are large enough to answer these questions.
To answer these questions, we formed the Breast Cancer Collaborative Group, a pooling project involving 20 prospective cohort studies. We included 890,000 women from 15 of these long-term studies across three continents, including over 18,000 incident breast cancer cases.
MedicalResearch.com: What are the main findings?
Response: We found that compared to women who never had a child, women who gave birth had an increased risk for breast cancer that peaked at 80% approximately 5 years after pregnancy, but lasted for, on average, 24 years before pregnancy became protective. The association between recent pregnancy and breast cancer was stronger for women who were older at first birth, had more births, or had a family history of breast cancer. The risk pattern was the same whether or not women breastfed.
MedicalResearch.com: What should readers take away from your report?
Response: Information from this study should help women have discussions with their doctors about when to start breast cancer screening. For example, women who were older at first birth, gave birth after age 40, and have a family history of breast cancer may want to start screening before the currently recommended age of 50.
MedicalResearch.com: What recommendations do you have for future research as a result of this work?
Response: Recent pregnancy is another example of a risk factor that is different for younger or older women. There is a need to develop risk prediction tools that are specific to young women.
MedicalResearch.com: Is there anything else you would like to add?
Response: Women and doctors should keep these findings in perspective. We studied the relative risk – comparing the rate of breast cancer in women who did or did not have a recent birth. But, breast cancer is rare in women under the age of 55. The absolute difference in risk – that is, the number of additional cases due to pregnancy, is actually very small.
Hazel B. Nichols et al. Breast Cancer Risk After Recent Childbirth: A Pooled Analysis of 15 Prospective Studies. Annals of Internal Medicine, 2018 DOI: 10.7326/M18-1323
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