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History of breast feeding and risk of incident endometriosis: prospective cohort study

BMJ 2017; 358 doi: https://doi.org/10.1136/bmj.j3778 (Published 29 August 2017) Cite this as: BMJ 2017;358:j3778
  1. Leslie V Farland, research scientist1 2,
  2. A Heather Eliassen, associate professor1 3,
  3. Rulla M Tamimi, associate professor1 3,
  4. Donna Spiegelman, professor1 3,
  5. Karin B Michels, professor and chair1 4,
  6. Stacey A Missmer, professor1 5 6
  1. 1Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA 02115, USA
  2. 2Center for Infertility and Reproductive Surgery, Department of Obstetrics, Gynecology, and Reproductive Biology, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02115, USA
  3. 3Channing Division of Network Medicine, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA 02115, USA
  4. 4Department of Epidemiology, Fielding School of Public Health, University of California, Los Angeles, CA, USA
  5. 5Division of Adolescent and Young Adult Medicine, Department of Medicine, Boston Children’s Hospital and Harvard Medical School, Boston, MA 02115, USA
  6. 6Department of Obstetrics, Gynecology, and Reproductive Biology, College of Human Medicine, Michigan State University, Grand Rapids Michigan, MI 49503, USA
  1. Correspondence to: L V Farland lfarland{at}mail.harvard.edu
  • Accepted 4 August 2017

Abstract

Objective To investigate the association between lifetime breast feeding, exclusive breast feeding, postpartum amenorrhea, and incidence of endometriosis among parous women.

Design Prospective cohort study.

Setting Nurses’ Health Study II, 1989-2011.

Participants 72 394women who reported having one or more pregnancies that lasted at least six months, 3296 of whom had laparoscopically confirmed endometriosis. For each pregnancy, women reported duration of total breast feeding, exclusive breast feeding, and postpartum amenorrhea.

Main outcome measures Incident self reported laparoscopically confirmed endometriosis (96% concordance with medical record) in parous women. Multivariable Cox proportional hazard models were used to calculate hazard ratios and 95% confidence intervals for diagnosis of endometriosis.

Results Duration of total and exclusive breast feeding was significantly associated with decreased risk of endometriosis. Among women who reported a lifetime total length of breast feeding of less than one month, there were 453 endometriosis cases/100 000 person years compared with 184 cases/100 000 person years in women who reported a lifetime total of ≥36 months of breast feeding. For every additional three months of total breast feeding per pregnancy, women experienced an 8% lower risk of endometriosis (hazard ratio 0.92, 95% confidence interval 0.90 to 0.94; P<0.001 for trend) and a 14% lower risk for every additional three months of exclusive breast feeding per pregnancy (0.86, 0.81 to 0.90; P<0.001 for trend). Women who breastfed for ≥36 months in total across their reproductive lifetime had a 40% reduced risk of endometriosis compared with women who never breast fed (0.60, 0.50 to 0.72). The protective association with breast feeding was strongest among women who gave birth within the past five years (P=0.04 for interaction). The association with total breast feeding and exclusive breast feeding on endometriosis was partially influenced by postpartum amenorrhea (% mediated was 34% (95% confidence interval 15% to 59%) for total breast feeding and 57% (27% to 82%) for exclusive breast feeding).

Conclusion Among women who experienced at least one pregnancy that lasted at least six months, breast feeding was inversely associated with risk of incident endometriosis. This association was partially, but not fully, influenced by postpartum amenorrhea, suggesting that breast feeding could influence the risk of endometriosis both through amenorrhea and other mechanisms. Given the chronic and incurable nature of endometriosis, breast feeding should be further investigated as an important modifiable behavior to mitigate risk for pregnant women.

Footnotes

  • We thank the participants and staff of the Nurses’ Health Study II for their valuable contributions. The authors assume full responsibility for analyses and interpretation of these data. An abstract of this work was presented as an oral presentation at the annual meeting of the 13th World Congress on Endometriosis in May 2017.

  • Contributors: LVF, SAM, RMT, AHE, and DS were responsible for study concept and design. LVF analyzed the data and wrote the manuscript, while all authors critically reviewed the manuscript and approved the final version. All authors also had full access to all of the data (including statistical reports and tables) in the study and can take responsibility for the integrity of the data and the accuracy of the data analysis. LVF is guarantor.

  • Funding: This work was supported by National Institutes of Health, National Cancer Institute (UM1 CA176726, 3R25CA057711 to LVF), National Institute of Child Health and Human Development (HD57210, T32HD060454 to LVF), and the Dana Farber and Harvard Cancer Center Mazzone Award. The NIHR had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The views expressed in this publication are those of the authors and not necessarily those of the funding agencies.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: support from the Harvard T H Chan School of Public Health, Eunice Kennedy Shriver National Institute of Child Health and Human Development, and National Cancer Institute for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: The study was approved by the institutional review board at Brigham and Women’s Hospital and Harvard medical school.

  • Data sharing: No additional data available.

  • Transparency: The lead author affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.

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