Contraceptive Efficacy of Emergency Contraception With Levonorgestrel Given Before or After Ovulation

Contraception. 2010 May;81(5):414-20. doi: 10.1016/j.contraception.2009.12.015. Epub 2010 Jan 27.

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.

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.

Publication types

  • Clinical Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Coitus
  • Contraception, Postcoital*
  • Contraceptive Agents, Female / administration & dosage*
  • Female
  • Humans
  • Levonorgestrel / administration & dosage*
  • Menstrual Cycle*
  • Ovulation*
  • Prospective Studies
  • Young Adult

Substances

  • Contraceptive Agents, Female
  • Levonorgestrel