title: Estimating the effectiveness of emergency contraceptive pills.
abstract: OBJECTIVE
  We use new estimates of conception probabilities by cycle day of intercourse, where cycle day is measured with day 1 being the first day of bleeding in a cycle, to propose a new approach for estimating the effectiveness of emergency contraceptive pills (ECPs). We use this new approach to examine the absolute effectiveness and the cost-effectiveness of ECPs and whether ECPs are more effective the sooner after unprotected intercourse they are initiated.
  
  
  METHODS
  Using the new set of conception probabilities, we employ data from two recent clinical trials of ECPs, one from the Population Council and the other from the World Health Organization (WHO), to examine the effectiveness of the combined ECP regimen.
  
  
  RESULTS
  The expected pregnancy rate among typical users was 6.2% in the Population Council trial and 7.4% in the WHO trial based on conception probabilities by cycle day relative to the day of ovulation. Based on conception probabilities by cycle day relative to the first day of bleeding, the expected pregnancy rates dropped to 5.4% and 5.2%, respectively. The two trials yield conflicting evidence regarding whether effectiveness declines with treatment delay.
  
  
  CONCLUSIONS
  Our results suggest that the absolute levels of effectiveness for the Yuzpe regimen of emergency contraception and the cost-effectiveness of this regimen have probably been overstated when based on conception probabilities by cycle day relative to day of ovulation.
authors.affiliation: Office of Population Research, Wallace Hall, Princeton University, Princeton, NJ 08544, USA. trussell@princeton.edu
authors.forename: James
authors.initials: J
authors.lastname: Trussell
authors.forename: Charlotte
authors.initials: C
authors.lastname: Ellertson
authors.forename: Helena
authors.initials: H
authors.lastname: von Hertzen
authors.forename: Allison
authors.initials: A
authors.lastname: Bigrigg
authors.forename: Anne
authors.initials: A
authors.lastname: Webb
authors.forename: Margaret
authors.initials: M
authors.lastname: Evans
authors.forename: Sue
authors.initials: S
authors.lastname: Ferden
authors.forename: Clare
authors.initials: C
authors.lastname: Leadbetter
mesh_terms: D000328:Adult; D002986:Clinical Trials as Topic; D003075:Coitus / Q000502:physiology*; D003282:Contraceptives, Postcoital, Hormonal / Q000191:economics / Q000627:therapeutic use*; D003362:Cost-Benefit Analysis; D004630:Emergencies; D005260:Female; D006801:Humans; D008597:Menstrual Cycle / Q000502:physiology*; D010060:Ovulation / Q000502:physiology; D011247:Pregnancy; D013223:Statistics as Topic; D013997:Time Factors
chemical_list: D003282:Contraceptives, Postcoital, Hormonal