title: Effect on pregnancy rates of the delay in the administration of levonorgestrel for emergency contraception: a combined analysis of four WHO trials.
abstract: BACKGROUND
  Levonorgestrel is an effective method for emergency contraception (EC) and is used worldwide. Consistent with its mechanism of action in delaying ovulation, the earlier it is administered within 72 h of an unprotected act of intercourse, the more effective it is. There is uncertainty, however, about its effectiveness after 72 h. This analysis explores the effect of 24-h intervals of delay in levonorgestrel administration on pregnancy rates when used until 120 h of an unprotected act of intercourse.
  
  
  METHODS
  Data were analyzed from 6794 women participating in four World Health Organization randomized trials and receiving 1.5 mg of levonorgestrel for EC in a single dose or split into two doses 12 h apart, within 48, 72 or 120 h of an act of unprotected intercourse. The pregnancy rates among women in successive days after an unprotected act of intercourse and odds ratios of pregnancy were calculated using logistic regression with the first day as the reference.
  
  
  RESULTS
  For the four trials combined, odds ratios for pregnancy in the second, third and fourth day with respect to the first day were not significantly different from 1 at the 5% level of significance. On the fifth day, the odds ratio of pregnancy compared to the first day was almost 6.
  
  
  CONCLUSIONS
  Levonorgestrel for EC should be administered as soon as possible after unprotected intercourse. Delaying levonorgestrel administration until the fifth day after unprotected intercourse increases the risk of pregnancy over five times compared with administration within 24 h. It is uncertain whether levonorgestrel administration on the fifth day still offers some protection against unwanted pregnancy.
authors.affiliation: Statistika Consultoria, São Paulo, Brazil. gilda.piaggio@gmail.com
authors.forename: Gilda
authors.initials: G
authors.lastname: Piaggio
authors.forename: Nathalie
authors.initials: N
authors.lastname: Kapp
authors.forename: Helena
authors.initials: H
authors.lastname: von Hertzen
mesh_terms: D044363:Contraception, Postcoital; D003271:Contraceptive Agents, Female / Q000008:administration & dosage*; D005260:Female; D006801:Humans; D016912:Levonorgestrel / Q000008:administration & dosage*; D011247:Pregnancy; D018873:Pregnancy Rate; D016032:Randomized Controlled Trials as Topic; D013997:Time Factors; D014944:World Health Organization
chemical_list: D003271:Contraceptive Agents, Female; D016912:Levonorgestrel