title: Menstrual cycle characteristics and incidence of premenopausal breast cancer.
abstract: BACKGROUND
  Epidemiologic studies have indicated that menstrual cycle characteristics such as age at menarche and age at menopause are associated with breast cancer risk. Anovulation, which is more common with long or irregular cycles, has been hypothesized to reduce the incidence of breast cancer.
  
  
  METHODS
  We analyzed data from the Nurses' Health Study II, a cohort of 116,671 female registered nurses ages 25 to 42 years at baseline. Information on menstrual cycle characteristics was assessed in 1989 and 1993, and incident cases of premenopausal breast cancer were ascertained through 2001.
  
  
  RESULTS
  During 1,135,496 person-years of follow-up (1989-2001), 1,163 incident cases of invasive premenopausal breast cancer were diagnosed. Overall, women with long menstrual cycles at ages 18 to 22 years (>32 days or too irregular to estimate) did not experience a significantly lower breast cancer risk compared with women with normal cycle lengths (26-31 days) at that age [covariate-adjusted hazard ratio (HR), 0.92; 95% confidence interval (95% CI), 0.79-1.06]. Among women ages <40 years, those with menstrual cycles lasting >32 days or too irregular to estimate at ages 18 to 22 years had a decreased incidence of breast cancer (covariate-adjusted HR, 0.71; 95% CI, 0.53-0.97). Current menstrual cycle characteristics were not associated with breast cancer incidence.
  
  
  CONCLUSIONS
  Overall, longer or irregular cycles at ages 18 to 22 years or in early adulthood were not associated with reduced premenopausal breast cancer risk. However, longer menstrual cycles at ages 18 to 22 years were associated with a lower incidence of premenopausal breast cancer before age 40.
authors.affiliation: Obstetrics and Gynecology Epidemiology Center, Harvard School of Public Health, Brigham and Women's Hospital, 221 Longwood Avenue, Boston, Massachusetts 02115, USA. kterry@hsph.harvard.edu
authors.forename: Kathryn L
authors.initials: KL
authors.lastname: Terry
authors.forename: Walter C
authors.initials: WC
authors.lastname: Willett
authors.forename: Janet W
authors.initials: JW
authors.lastname: Rich-Edwards
authors.forename: David J
authors.initials: DJ
authors.lastname: Hunter
authors.forename: Karin B
authors.initials: KB
authors.lastname: Michels
mesh_terms: D000328:Adult; D017668:Age of Onset; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008598:Menstruation / Q000502:physiology*; D008599:Menstruation Disturbances / Q000150:complications*; D017697:Premenopause; D012307:Risk Factors