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CPC-023 Assessment of the Whole Interceptive and Post-Fertilisation Effects of Postcoital Levonorgestrel
  1. EJ Alegre-del Rey,
  2. S Fénix-Caballero,
  3. JF López-Vallejo,
  4. MJ Ladrón de Guevara,
  5. MA Blanco-Castaño,
  6. E Ríos-Sánchez,
  7. JM Borrero-Rubio
  1. Hospital Universitario Puerto Real, Pharmacy, Puerto Real (Cádiz), Spain

Abstract

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 als data, as they observed only 35 women.

No conflict of interest.

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