title: Postmenopausal breast cancer risk and cumulative number of menstrual cycles.
abstract: OBJECTIVE
  To explore whether the lifetime cumulative number of menstrual cycles, as an index for total exposure to endogenous estrogens, and the number of menstrual cycles until a first full-term pregnancy (FFTP), are associated with breast cancer risk in postmenopausal women.
  
  
  METHODS
  Population-based study with data from the Prospect-European Prospective Investigation into Cancer and Nutrition study. Naturally menopausal participants were eligible (n = 6,718). The cumulative number of menstrual cycles was computed in 6,031 (90%) women. We calculated the number of cycles until FFTP among parous participants. The number of menstrual cycles was impossible to compute in women who reported to be always irregular; therefore, we added the "always irregular" category in the analysis. During the 46,746 person-years of follow-up, 168 breast cancer cases were identified. Cox regression models were used and adjustments were made to account for potential confounders.
  
  
  RESULTS
  Even when our data does not show a clear linear gradient, we observed an increased breast cancer risk in women with a higher number of cumulative menstrual cycles in their lifetime. Using < or = 415 cycles as reference, the hazard ratio for the irregular group, 416-453, 454-490, and > or = 491 cycles was 1.11 (.56, 2.19), 1.88 (1.14, 3.12), 1.74 (1.05, 2.87), and 1.80 (1.09, 2.96), respectively. Although not statistically significant, and of less magnitude, the risk estimates for the number of cycles before FFTP showed the same tendency.
  
  
  CONCLUSIONS
  Among women who underwent natural menopause, a higher number of menstrual cycles in lifetime, reflecting a longer exposure to endogenous estrogens, is associated with an increased breast cancer risk.
authors.affiliation: Julius Center for Health Sciences and Primary Care, University Medical Center, Room STR 6.1.31, P.O. Box 85060, 3508 BA Utrecht, The Netherlands.
authors.forename: Mariana
authors.initials: M
authors.lastname: Chavez-MacGregor
authors.forename: Sjoerd G
authors.initials: SG
authors.lastname: Elias
authors.forename: N Charlotte
authors.initials: NC
authors.lastname: Onland-Moret
authors.forename: Yvonne T
authors.initials: YT
authors.lastname: van der Schouw
authors.forename: Carla H
authors.initials: CH
authors.lastname: Van Gils
authors.forename: Evelyn
authors.initials: E
authors.lastname: Monninkhof
authors.forename: Diederick E
authors.initials: DE
authors.lastname: Grobbee
authors.forename: Petra H M
authors.initials: PH
authors.lastname: Peeters
mesh_terms: D000368:Aged; D001943:Breast Neoplasms / Q000209:etiology*; D015331:Cohort Studies; D005060:Europe; D005260:Female; D006801:Humans; D008572:Menarche; D008597:Menstrual Cycle; D008875:Middle Aged; D015337:Multicenter Studies as Topic; D011159:Population Surveillance / Q000379:methods*; D017698:Postmenopause; D012307:Risk Factors