Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation.
BACKGROUND
The contraceptive efficacy of emergency contraceptive pills containing levonorgestrel (LNG-EC) has been estimated in most previous studies by judging the day of ovulation from presumptive menstrual cycle data, thus providing poorly reliable estimates.


METHODS
In the present study, the efficacy of LNG-EC was determined in 393 cycles by dating ovulation on the basis of reliable hormonal and ovarian parameters validated by a database constructed in a separate study. In addition, the efficacy was determined separately for cycles in which LNG-EC was given before or after ovulation.


RESULTS
For the 148 women who had sexual intercourse during the fertile days, the overall accumulated probability of pregnancy was 24.7, while altogether 8 pregnancies were observed. Thus, the overall contraceptive efficacy of LNG-EC was 68%. Among the 103 women who took LNG-EC before ovulation (days -5 to -1), 16 pregnancies were expected and no pregnancy occurred (p<.0001). Among the 45 women who took LNG-EC on the day of ovulation (day 0) or thereafter, 8 pregnancies occurred and 8.7 were expected (p=1.00). These findings are incompatible with the inhibition of implantation by LNG-EC in women. The same cases were also analyzed using the presumptive menstrual cycle data, and important discrepancies were detected between the two methods.


CONCLUSIONS
The efficacy of LNG-EC has been overestimated in studies using presumptive menstrual cycle data. Our results confirm previous similar studies and demonstrate that LNG-EC does not prevent embryo implantation and therefore cannot be labeled as abortifacient.
Instituto Chileno de Medicina Reproductiva, Santiago, Chile. gnoe@icmer.org
Gabriela
G
Noé
Horacio B
HB
Croxatto
Ana María
AM
Salvatierra
Verónica
V
Reyes
Claudio
C
Villarroel
Carla
C
Muñoz
Gabriela
G
Morales
Anita
A
Retamales
D000293:Adolescent; D000328:Adult; D003271:Contraceptive Agents, Female / Q000008:administration & dosage*; D003280:Contraceptives, Oral, Synthetic / Q000008:administration & dosage*; D003281:Contraceptives, Postcoital / Q000008:administration & dosage*; D004638:Emergency Treatment; D005260:Female; D006801:Humans; D016912:Levonorgestrel / Q000008:administration & dosage*; D008597:Menstrual Cycle; D010060:Ovulation; D011247:Pregnancy; D011256:Pregnancy Outcome; D055815:Young Adult
D003271:Contraceptive Agents, Female; D003280:Contraceptives, Oral, Synthetic; D003281:Contraceptives, Postcoital; D016912:Levonorgestrel