Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation

Gabriela Noé, Horacio B. Croxatto, Ana María Salvatierra, Verónica Reyes, Claudio Villarroel, Carla Muñoz, Gabriela Morales, Anita Retamales

Research output: Contribution to journalArticlepeer-review

64 Citations (Scopus)


Background: We aimed to evaluate whether emergency contraception with levonorgestrel (LNG-EC) administered after ovulation is equally effective to LNG-EC administered before ovulation. Study design: 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. Conclusion: We conclude that LNG-EC prevents pregnancy only when taken before fertilization of the ovum has occurred.

Original languageEnglish
Pages (from-to)414-420
Number of pages7
Issue number5
Publication statusPublished - May 2010


  • Contraceptive efficacy
  • Emergency contraception
  • Levonorgestrel

ASJC Scopus subject areas

  • Reproductive Medicine
  • Obstetrics and Gynaecology


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