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

• Published in: Contraception (Stoneham) 2010-05, Vol.81 (5), p.414-420
• Main Authors: Noé, Gabriela, Croxatto, Horacio B, Salvatierra, Ana María, Reyes, Verónica, Villarroel, Claudio, Muñoz, Carla, Morales, Gabriela, Retamales, Anita
• Format: Article
• Language: English
• Subjects: Adolescent; Adult; Biological and medical sciences; Birth control; Coitus; Contraception, Postcoital; Contraceptive Agents, Female - administration & dosage; Contraceptive efficacy; Emergency contraception; Female; Genital system. Reproduction; Gynecology. Andrology. Obstetrics; Hormonal contraception; Humans; Levonorgestrel; Levonorgestrel - administration & dosage; Medical sciences; Menstrual Cycle; Obstetrics and Gynecology; Ovulation; Pharmacology. Drug treatments; Prospective Studies; Young Adult
• ISSN: 0010-7824; 1879-0518
• DOI: 10.1016/j.contraception.2009.12.015

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.