Abstract
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.
Conclusion
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.
Keywords
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Article info
Publication history
Published online: April 27, 2011
Accepted:
March 9,
2011
Received in revised form:
March 7,
2011
Received:
October 22,
2010
Identification
Copyright
© 2011 Elsevier Inc. Published by Elsevier Inc. All rights reserved.
