Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation.
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.
Noé
Gabriela
G
Instituto Chileno de Medicina Reproductiva (ICMER), Santiago, Chile. gnoe@icmer.org
Croxatto
Horacio B
HB
Salvatierra
Ana María
AM
Reyes
Verónica
V
Villarroel
Claudio
C
Muñoz
Carla
C
Morales
Gabriela
G
Retamales
Anita
A
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
D003271:Contraceptive Agents, Female; D016912:Levonorgestrel