title: Dual effect of parity on breast cancer risk in African-American women.
abstract: BACKGROUND
  In the United States, breast cancer incidence is higher among African-American women than among white women before age 45 but lower at older ages. To explore whether differences in childbearing patterns can explain this observation, we assessed the relation of several childbearing variables to breast cancer risk in a large prospective cohort study of U.S. African-American women.
  
  
  METHODS
  Black Women's Health Study participants were enrolled in 1995 and were followed by mailed questionnaires every 2 years (in 1997 and 1999). Of the 64,500 women enrolled, 56,725 (88%) completed at least one of the follow-up questionnaires. During 214,862 person-years of follow-up, participants reported 349 breast cancers, of which 128 were among women younger than 45 years and 221 were among women aged 45-70 years. Incidence rate ratios (IRRs) with 95% confidence intervals (CIs) were derived from age-stratified Cox regression models that adjusted for each of the childbearing variables (parity, age at first birth, and time since last birth).
  
  
  RESULTS
  Compared with primiparity, high parity was associated with an increased risk of breast cancer among women younger than 45 years (IRR for four or more births = 2.4, 95% CI = 1.1 to 5.1) and a decreased risk among women aged 45 years and older (IRR = 0.5, 95% CI = 0.3 to 0.9). The IRR for late age at first birth compared with early age was 2.5 (95% CI = 1.1 to 5.8) among the younger women and was not elevated among older women. We found no statistically significant association of time since last birth with breast cancer risk among either younger or older women.
  
  
  CONCLUSIONS
  Parity has a dual association with breast cancer risk in African-American women; among women younger than 45 years, parity is associated with an increased risk and among women 45 years and older it is associated with a decreased risk. This dual effect may explain some of the observed differences in breast cancer incidence rates among African-American and white women.
authors.affiliation: Slone Epidemiology Center, Boston University, Boston, MA 02215, USA. jpalmer@slone.bu.edu
authors.forename: Julie R
authors.initials: JR
authors.lastname: Palmer
authors.forename: Lauren A
authors.initials: LA
authors.lastname: Wise
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Horton
authors.forename: Lucile L
authors.initials: LL
authors.lastname: Adams-Campbell
authors.forename: Lynn
authors.initials: L
authors.lastname: Rosenberg
mesh_terms: D000328:Adult; D001741:Black or African American / Q000706:statistics & numerical data*; D017677:Age Distribution; D000367:Age Factors; D000368:Aged; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology / Q000517:prevention & control; D015331:Cohort Studies; D016001:Confidence Intervals; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D016017:Odds Ratio; D010298:Parity; D018570:Risk Assessment; D012307:Risk Factors; D011795:Surveys and Questionnaires; D014481:United States / Q000453:epidemiology