Abstract
Emergency post-coital contraceptives effectively reduce the risk of pregnancy, but
their degree of efficacy remains uncertain. Measurement of efficacy depends on the
pregnancy rate without treatment, which cannot be measured directly. We provide indirect
estimates of such pregnancy rates, using data from a prospective study of 221 women
who were attempting to conceive. We previously estimated the probability of pregnancy
with an act of intercourse relative to ovulation. In this article, we extend these
data to estimate the probability of pregnancy relative to intercourse on a given cycle
day (counting from onset of previous menses). In assessing the efficacy of post-coital
contraceptives, other approaches have not incorporated accurate information on the
variability of ovulation. We find that the possibility of late ovulation produces
a persistent risk of pregnancy even into the sixth week of the cycle. Post-coital
contraceptives may be indicated even when intercourse has occurred late in the cycle.
Keywords
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Copyright
© 2001 Elsevier Science Inc. Published by Elsevier Inc. All rights reserved.
