title: Prepregnancy exposure to cigarette smoking and subsequent risk of postmenopausal breast cancer.
abstract: OBJECTIVE
  To examine the association of cigarette smoking before first pregnancy with risk of postmenopausal breast cancer in a large population-based cohort.
  
  
  METHODS
  The Iowa Women's Health Study is a prospective cohort study of 55- to 69-year-old women at baseline in 1986. In January 1986, a questionnaire was mailed to 99,826 postmenopausal women to Identify risk factors for cancer and other chronic diseases; 41,836 women responded (42.7% response rate). The primary analyses examined the associations among smoking, parity, age at first birth, and postmenopausal breast cancer.
  
  
  RESULTS
  Of the 37,105 women in the cohort at risk, 7095 (19%) and 4186 (11%) initiated smoking before and after first pregnancy, respectively, and 2017 breast cancers were identified before December 31, 1999. Compared with parous women who never smoked, women who began smoking after their first full-term pregnancy were not at increased risk of postmenopausal breast cancer (multivariate-adjusted risk ratio, 1.03; 95% confidence interval, 0.88-1.21). However, women who began smoking before their first pregnancy had a slightly elevated risk of breast cancer (risk ratio, 1.21; 95% confidence Interval, 1.07-1.37). Results were not attenuated by adjustment for age at first pregnancy or number of live births.
  
  
  CONCLUSIONS
  These data suggest that cigarette smoking is associated with a slightly greater risk of postmenopausal breast cancer for women who started smoking before their first full-term pregnancy.
authors.affiliation: Division of Epidemiology, Mayo Clinic College of Medicine, Rochester, MN 55905, USA. olsonj@mayo.edu
authors.forename: Janet E
authors.initials: JE
authors.lastname: Olson
authors.forename: Celine M
authors.initials: CM
authors.lastname: Vachon
authors.forename: Robert A
authors.initials: RA
authors.lastname: Vierkant
authors.forename: Carol
authors.initials: C
authors.lastname: Sweeney
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Limburg
authors.forename: James R
authors.initials: JR
authors.lastname: Cerhan
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Sellers
mesh_terms: D000367:Age Factors; D000368:Aged; D001943:Breast Neoplasms / Q000209:etiology*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008875:Middle Aged; D017698:Postmenopause; D017584:Reproductive History; D018570:Risk Assessment; D012907:Smoking / Q000009:adverse effects*