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Editorials |1 January 2019

Learning From Temporal Relationships: Childbirth and Breast Cancer Risk

Katrina Armstrong, MD, MSCE

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This article was published at Annals.org on 11 December 2018.
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The relationship between childbirth and breast cancer risk is complex. Childbirth is an intrinsically multidimensional “exposure,” and understanding its effects requires disentangling age, parity, use of contraceptive and reproductive technology, and breastfeeding, among other factors. Given this complexity, the results of the large and sophisticated analysis by Nichols and colleagues (1) provide an important advance in this field. In this examination of nearly 900 000 women aged younger than 55 years who were enrolled in studies of premenopausal breast cancer, risk for breast cancer peaked 5 years after childbirth and then gradually decreased for at least 2 decades. Because of the large study population, the investigators could also examine important subgroups: The effect was not seen for women who had only 1 child or had their first child before age 25 years, and it was greatest for estrogen receptor–negative cancer and among women with a family history of breast cancer.
Before considering the potential implications, we must recognize that these results come with several caveats. A randomized trial of time since childbirth is not feasible, and proving causality in an observational study remains challenging. The investigators made considerable analytic effort to isolate the effect of time since childbirth from that of other factors; however, the analyses are limited to the data collected, and the contribution of unmeasured factors (such as weight gain and breast cancer screening) to the observed association cannot be addressed. Because the most straightforward comparison reported is that of uniparous versus nulliparous women, the absence...
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Armstrong K. Learning From Temporal Relationships: Childbirth and Breast Cancer Risk. Ann Intern Med. 2019;170:66–67. [Epub ahead of print 11 December 2018]. doi: https://doi.org/10.7326/M18-3455

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Published: Ann Intern Med. 2019;170(1):66-67.

DOI: 10.7326/M18-3455

Published at www.annals.org on 11 December 2018

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2018 American College of Physicians
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