Background Taking into account the whole interceptive effect, anovulatory potency and timing of administration, it’s possible to calculate what proportion of interceptive (contraceptive±contragestive) effects of levonorgestrel take place as anovulatory action. However, we don’t know the actual interceptive effect, because clinical trials didn’t use a placebo group.
Purpose To discover the interceptive effect after a single dose of levonorgestrel, and then calculating the proportion of its anovulatory and possible post-fertilisation effects.
Materials and Methods A recent systematic review pulled data from 6,794 women. Levonorgestrel administered the fifth day after intercourse showed a probability of pregnancy of 5.2%, slightly lower than the 6–8% calculated by an estimation method. Using this cohort as a control group, we estimated the interceptive effect and extrapolated it in Mikolajczyk & Stanford’s graph (2007) to find out what proportions result from anovulatory or post-fertilisation effects.
Results The pregnancy rate was 1.0% taking the pill 1–4 days after intercourse (66 pregnancies in 6,564 women), and 5.2% if it was taken on the fifth day (12 in 230 women). It shows a minimum reduction in the probability of pregnancy of 80.7% (IC95 64.9–89.4%).
In a conservative approach, administering the pill 24 h after intercourse, we obtained an anovulatory effect of 50%. However, taking into account epidemiological data showing lack of effect on pregnancy rates at a population level, we could assume an actual decrease that could be in the lower top of the confidence interval (64.9%). Extrapolating this effect, we obtained a contribution of 65% for the anovulatory mechanism.
Conclusions As an alternative pre-fertilisation effect is unlikely, we postulate at least 35% post-fertilisation effects for post-coital levonorgestrel. This is statistically compatible with the previous contradictory Noe et al’s data, as they observed only 35 women.
No conflict of interest.
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