American Journal of Epidemiology Copyright ª 2007 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.

Vol. 165, No. 7 DOI: 10.1093/aje/kwk068 Advance Access publication January 10, 2007

Original Contribution Oral Contraceptives, Reproductive Factors, and Risk of Colorectal Cancer among Women in a Prospective Cohort Study

Jennifer Lin1, Shumin M. Zhang1,2, Nancy R. Cook1,2, JoAnn E. Manson1,2,3, Julie E. Buring1,2,4, and I-Min Lee1,2 Division of Preventive Medicine, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA. 2 Department of Epidemiology, Harvard School of Public Health, Boston, MA. 3 Channing Laboratory, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA. 4 Department of Ambulatory Care and Prevention, Harvard Medical School, Boston, MA. Received for publication March 31, 2006; accepted for publication September 18, 2006.

Previous findings on the associations between oral contraceptive (OC) use and reproductive factors and risk of colorectal cancer have been inconclusive. The authors evaluated associations of OC use and reproductive factors (including parity, age at first birth, age at menarche, and age at menopause) with risk of colorectal cancer among women in a large-scale prospective cohort study. The analysis included 39,680 participants in the Women’s Health Study who had usable information on ever use of OCs and potential risk factors for colorectal cancer. Relative risks and 95% confidence intervals were estimated from Cox proportional hazards regression models. All p values were two-sided. During an average of 11 years of follow-up (1992–2004), 267 incident cases of colorectal cancer were documented. Ever use of OCs was associated with a lower risk of colorectal cancer (relative risk ¼ 0.67, 95% confidence interval: 0.50, 0.89). Women who had used OCs for 6 months–