After receiving a consultation from a patient who, after ingesting a dose of postcoital contraceptive, has regretted the possibility that it acts through an anti-implantation mechanism, which would cause the death of the early embryo. We proceed to propose a reflection on this possibility.


Official bodies such as the WHO or FIGO maintain the impossibility of these drugs to interfere with the implantation of the early embryo, as stated in the package insert for postcoital contraceptives, and they justify their effectiveness as contraceptives in the mere ability to inhibit or delay ovulation.

But many studies disagree with this position, showing that the effectiveness of these drugs is mainly due to their ability to interfere with the implantation process, causing the death of the early embryo.

The extension of postcoital contraception raises new dilemmas such as the one that concerns us in this analysis: given the possibility that the woman who has ingested the pill will regret it, is it possible to intervene to avoid the possible anti-implantation effect? And the death of the embryo?

Mechanism of action of postcoital contraceptives

The two most used drugs in postcoital contraception are levonorgestrel and ulipristal. Its mechanism of action has been analyzed in our Observatory in previous reports.

The first is a molecule with effects analogous to progesterone, but more powerful than it. It shows its ability to inhibit ovulation if administered 5 or 4 days before it occurs or to interfere with implantation when ingested one of the three days prior to ovulation, lacking effectiveness if administered later.

The second, ulipristal acetate, is a selective inhibitor of progesterone receptors, and acts by preventing the progression of pregnancy, but it can also act by inhibiting ovulation if administered 5 or 4 days before it occurs.

Administered from the third day before ovulation and in the days after ovulation, it acts by interfering with the implantation process. This ability to continue acting as a contraceptive if administered after ovulation makes its use authorized for 5 days after unprotected sexual intercourse. This differentiates it from levonorgestrel, whose effectiveness is reduced to an interval of 3 days.

It should be noted that both drugs lack contraceptive efficacy if they are administered before the fifth day prior to ovulation.

Chemical abortifacients: mifepristone

Mifepristone, also known as RU-486, is a drug authorized for the practice of chemical abortions in many countries. It is used alone or, preferably, combined with misoprostol, a prostaglandin that increases its effectiveness as an abortifacient. Mifepristone and ulipristal molecules have very similar chemical structures, sharing a similar mechanism of action, based on their ability to block progesterone receptors, a hormone crucial for maintaining pregnancy.

Many studies have been previously published that analyze the possibility of reversing the abortive effect of mifepristone in cases where the woman who has ingested it regrets it and decides to continue with her pregnancy.

There is sufficient scientific information that supports this possibility through the administration of high doses of progesterone. But only in cases where it has not been administered together with misoprostol and during the first 7 weeks of pregnancy.

Reversal of the effects of postcoital contraceptives

The experience accumulated in the case of the abortifacient mifepristone allows us to wonder if it would be possible to do the same in the case of women who regret having administered a postcoital contraceptive.

The peculiarity of the mechanisms of action of postcoital contraceptives makes it difficult to determine whether this would be possible.

Credit: Robin Marty – flickr

There is no scientific evidence of the possibility of reversing the effects of levonorgestrel administered in emergency contraception. Its very limited period of effectiveness, 5 days prior to ovulation, leaves few options for reversing its effects. But in the case of ulipristal, a possibility of reversal could arise, which we will explain.

This molecule has chemical similarities with mifepristone and belongs to the same family of drugs known as “selective progesterone receptor modulators.” Its mechanism of action consists of blocking these receptors, hindering the action of progesterone, both in the pre- and post-ovulatory phase.

Unlike mifepristone, it is controversial whether or not ulipristal can interfere with the development of an already implanted embryo, but its ability to interfere with endometrial development and maturation, which translates into interfering with the progression of pregnancy, does seem to have been demonstrated.

The administration of progesterone in high doses has made it possible to reverse the abortive effect of mifepristone in many cases. This makes us wonder if it would also do so in the case of ulipristal when it has been administered in the three days prior to ovulation or after it has occurred, a period in which the drug shows its anti-implantation potential. If the administration is on the fifth or fourth day prior to ovulation, the mechanism of action of ulipristal is anovulatory.


Clinical studies could be developed to analyze the progression of pregnancies in two groups of women after administering ulipristal as postcoital contraception; the first would receive high-dose progesterone and the second a placebo. This study would demonstrate the ability of progesterone, administered orally or intramuscularly, to avoid the anti-implantation and lethal effect of ulipristal acetate for the embryo.

An unavoidable requirement to respect the free choice of women who, after making the decision to abort, have changed their mind, is to provide them with rigorous and sufficient information. This must be based on the available evidence, to make it easier for them to exercise their autonomy if it results in their child continuing to live.

Julio Tudela

Director of the Institute of Life Sciences

Catholic University of Valencia


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