title: Levonorgestrel emergency contraception: a joint analysis of effectiveness and mechanism of action.
abstract: OBJECTIVE
  To model the effectiveness that can be obtained if levonorgestrel-only emergency contraception (EC) acts only through disrupting ovulation, in relation to other effects that may occur before or after fertilization and accounting for delays in administration.
  
  
  METHODS
  We modeled follicular growth as a function of follicular size, using known day-specific probabilities of conception and known disruption of ovulation by levonorgestrel-only EC, to estimate the expected effectiveness of EC.
  
  
  METHODS
  Combined data from multiple clinical studies.
  
  
  METHODS
  Simulation models.
  
  
  METHODS
  Disruption of ovulation.
  
  
  METHODS
  Effectiveness in the form of proportion of pregnancies prevented.
  
  
  RESULTS
  With disruption of ovulation alone, the potential effectiveness of levonorgestrel EC ranged from 49% (no delay) to 8% (72-hour delay). With complete inhibition of fertilization before the day of ovulation, the potential effectiveness of levonorgestrel EC ranged from 90% (no delay) to 16% (72-hour delay).
  
  
  CONCLUSIONS
  The gap between effectiveness of levonorgestrel EC estimated from clinical studies and what can be attributed to disruption of ovulation may be explained by overestimation of actual effectiveness and supplementary mechanisms of action, including postfertilization effects. Additional data with follicular ultrasound and precise measures of delay between intercourse and EC administration would yield greater insight into effectiveness and mechanisms of action.
authors.affiliation: Department of Public Health Medicine, School of Public Health, University of Bielefeld, Bielefeld, Germany. rmikolajczyk@uni-bielefeld.de <rmikolajczyk@uni-bielefeld.de>
authors.forename: Rafael T
authors.initials: RT
authors.lastname: Mikolajczyk
authors.forename: Joseph B
authors.initials: JB
authors.lastname: Stanford
mesh_terms: D003280:Contraceptives, Oral, Synthetic / Q000008:administration & dosage*; D004630:Emergencies; D005260:Female; D005298:Fertility / Q000187:drug effects; D005306:Fertilization / Q000187:drug effects; D006801:Humans; D016912:Levonorgestrel / Q000008:administration & dosage*; D008597:Menstrual Cycle / Q000187:drug effects / Q000502:physiology; D006080:Ovarian Follicle / Q000187:drug effects / Q000502:physiology; D010060:Ovulation / Q000187:drug effects*
chemical_list: D003280:Contraceptives, Oral, Synthetic; D016912:Levonorgestrel