title: Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation.
abstract: BACKGROUND
  We aimed to evaluate whether emergency contraception with levonorgestrel (LNG-EC) administered after ovulation is equally effective to LNG-EC administered before ovulation.
  
  
  METHODS
  We studied a cohort of women attending a family planning clinic for EC. From interview, we recorded menstrual history, time of intercourse and of intake of LNG-EC. On the day of intake of LNG-EC and during 5 days' follow-up, blood samples were taken for examination of luteinizing hormone, estradiol and progesterone concentrations, and vaginal ultrasound examinations were done for size of the leading follicle and/or corpus luteum. Thereafter women were not contacted until next menses or pregnancy occurred.
  
  
  RESULTS
  Of 388 women attending for LNG-EC, 122 women had intercourse on fertile cycle days according to ultrasound and endocrine findings. At the time of LNG-EC intake, 87 women were in Days -5 to -1 and 35 women were in Day 0 (day of ovulation) or beyond. With the use of the probability of clinical pregnancy reported by Wilcox et al. [N Engl J Med 333 (1995) 1517-1521], expected numbers of pregnancies among the 87 and 35 women were 13 and 7, respectively, while 0 and 6 pregnancies, respectively, occurred.
  
  
  CONCLUSIONS
  We conclude that LNG-EC prevents pregnancy only when taken before fertilization of the ovum has occurred.
authors.affiliation: Instituto Chileno de Medicina Reproductiva (ICMER), Santiago, Chile. gnoe@icmer.org
authors.forename: Gabriela
authors.initials: G
authors.lastname: Noé
authors.forename: Horacio B
authors.initials: HB
authors.lastname: Croxatto
authors.forename: Ana María
authors.initials: AM
authors.lastname: Salvatierra
authors.forename: Verónica
authors.initials: V
authors.lastname: Reyes
authors.forename: Claudio
authors.initials: C
authors.lastname: Villarroel
authors.forename: Carla
authors.initials: C
authors.lastname: Muñoz
authors.forename: Gabriela
authors.initials: G
authors.lastname: Morales
authors.forename: Anita
authors.initials: A
authors.lastname: Retamales
mesh_terms: D000293:Adolescent; D000328:Adult; D003075:Coitus; D044363:Contraception, Postcoital; D003271:Contraceptive Agents, Female / Q000008:administration & dosage*; D005260:Female; D006801:Humans; D016912:Levonorgestrel / Q000008:administration & dosage*; D008597:Menstrual Cycle; D010060:Ovulation; D011446:Prospective Studies; D055815:Young Adult
chemical_list: D003271:Contraceptive Agents, Female; D016912:Levonorgestrel