title: Induced abortion and the risk of breast cancer.
abstract: BACKGROUND
  It has been hypothesized that an interrupted pregnancy might increase a woman's risk of breast cancer because breast cells could proliferate without the later protective effect of differentiation.
  
  
  METHODS
  We established a population-based cohort with information on parity and vital status consisting of all Danish women born from April 1, 1935, through March 31, 1978. Through linkage with the National Registry of Induced Abortions, information on the number and dates of induced abortions among those women was combined with information on the gestational age of each aborted fetus. All new cases of breast cancer were identified through linkage with the Danish Cancer Registry.
  
  
  RESULTS
  In the cohort of 1.5 million women (28.5 million person-years), we identified 370,715 induced abortions among 280,965 women (2.7 million person-years) and 10,246 women with breast cancer. After adjustment for known risk factors, induced abortion was not associated with an increased risk of breast cancer (relative risk, 1.00; 95 percent confidence interval, 0.94 to 1.06). No increases in risk were found in subgroups defined according to age at abortion, parity, time since abortion, or age at diagnosis of breast cancer. The relative risk of breast cancer increased with increasing gestational age of the fetus at the time of the most recent induced abortion: <7 weeks, 0.81 (95 percent confidence interval, 0.58 to 1.13); 7 to 8 weeks, 1.01 (0.89 to 1.14); 9 to 10 weeks, 1.00 >12 weeks, 1.38 (1.00 to 1.90) (reference category, 9 to 10 weeks).
  
  
  CONCLUSIONS
  Induced abortions have no overall effect on the risk of breast cancer.
authors.affiliation: Department of Epidemiology Research, Danish Epidemiology Science Center, Statens Serum Institut, Copenhagen, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Melbye
authors.forename: J
authors.initials: J
authors.lastname: Wohlfahrt
authors.forename: J H
authors.initials: JH
authors.lastname: Olsen
authors.forename: M
authors.initials: M
authors.lastname: Frisch
authors.forename: T
authors.initials: T
authors.lastname: Westergaard
authors.forename: K
authors.initials: K
authors.lastname: Helweg-Larsen
authors.forename: P K
authors.initials: PK
authors.lastname: Andersen
mesh_terms: D000028:Abortion, Induced / Q000009:adverse effects*; D000293:Adolescent; D000328:Adult; D001943:Breast Neoplasms / Q000453:epidemiology / Q000209:etiology*; D016022:Case-Control Studies; D002648:Child; D003718:Denmark / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D005865:Gestational Age; D006801:Humans; D008875:Middle Aged; D011247:Pregnancy; D012042:Registries; D017584:Reproductive History; D012306:Risk; D012307:Risk Factors