Plan B’s Main Mechanism of Action: The Case for a Post-Fertilization Effect: Symposium on Plan B: Part III

Editor’s Note: The editors present this series (read part 1, part 2, part 4, part 5, part 6)   on the recent furor over Plan B as an opportunity for our fellow pro-lifers to slow down, step out of activist mode, and enter into the conversation in a prayerful and thoughtful way.

The Church encourages conversation among faithful scientists and theologians as new science comes to light and as we deepen ethical reflection. Typically such conversations occur in academic settings, but since the recent furor pushed the issue into the open, causing much confusion and scandal, we felt it important to present the latest science and moral reflection in a context of faithful discussion.

We offer this series in a spirit of obedience to the Magisterium, and as an opportunity for faithful Catholics and people of good will to come to a greater understanding of the nuances of the Church’s teaching and the complexities of the science and art of medicine in the difficult situations involving the treatment of women who have been raped. There has been no (and will be no) revision in the Church’s teaching concerning direct abortion or contraceptive sexual acts between spouses. Both are morally illicit without exception.

As with all Truth and Charity Forum articles, opinions belong to the author alone and do not necessarily represent the official position of Human Life International.


There is much confusion surrounding how Plan B Emergency Contraception actually works. Plan B is a high dose progestin (levonorgestrel), which is designed to be taken up to 120 hours after intercourse to prevent pregnancy. Together with some other colleagues we have undertaken a review of the scientific literature on Plan B (recently submitted for publication) including all studies reportedly concluding that the drug worked primarily to inhibit or delay ovulation, and those that specifically concluded it had no post-fertilization effects. After re-analyzing each primary study, we arrived at a different conclusion.

We must recall that there are only six days during the woman’s cycle where she may become pregnant.[i] The five days before ovulation and the one day that the ovum survives after ovulation is known as the fertile window and occurs in the late follicular phase. The Plan B researchers identified the fertile window using urinary or serum gonadotropin (hormone) levels or transvaginal ultrasound to identify follicular rupture (FR), indicating that a woman had ovulated.

The largest study on Plan B by Gabriela Noé et al. showed that the majority of women ovulated (had documented FR) despite being given Plan B in their fertile window, yet there were no clinical pregnancies.[ii] Several other previous studies show similar findings.[iii]

When Plan B was given at or after ovulation, there was no difference between expected and observed pregnancies (i.e. the drug had no efficacy at all). Other researchers posited that if Plan B did not affect the number of pregnancies observed when given on or after ovulation, then no post-fertilization effect must therefore have been present.[iv]

But actually, all we can observe is that Plan B has no efficacy on the day of ovulation or afterwards. The more important finding involves the 80% of women who ovulated despite receiving Plan B during their fertile window. How did researchers say that the pill worked to prevent pregnancy in this scenario?

Previous review articles attributed some of Plan B’s efficacy to the drug’s effect on cervical mucus and sperm.[v] When levonorgestrel is used in regular continuous contraceptives, the drug does have effects on cervical mucus, inhibiting sperm from traveling to the egg that has been released. However, when it is given as a one-time Emergency Contraceptive, it has no such effect.[vi] Moreover, recent studies show that Plan B has no effects on cervical mucus, sperm function, or the ability of sperm to fertilize the egg.[vii]

Despite these facts, many researchers still claim that Plan B exerts these effects and quote research that is 40 years old.[viii] Moreover, if Plan B actually did have such a robust effect on sperm, one that did prevent sperm from fertilizing the ova, why is it that this mechanism of action would not work on the very fertile day of ovulation? However, there is absolutely no evidence at all for any Plan B efficacy when it is given on the day of ovulation or afterwards.

Since Plan B does not affect cervical mucus or sperm, and as many studies show sperm can reach the fallopian tubes within minutes of intercourse,[ix] and since ovulations are documented (by means of FR), and without any other barriers to fertilization, we must assume that the egg becomes fertilized, and that Plan B works after fertilization to prevent the embryo from surviving. Many of the same studies that were used to provide evidence regarding Plan B’s supposed predominant mechanism of action, suppression of ovulation, actually give us valuable information about a possible post-fertilization mechanism of action.

Thus, we see that when Plan B is given in the late follicular phase (the fertile part of the female cycle), it causes blunted LH surge, lowered progesterone levels, shortened luteal phase and increased vaginal bleeding. All of these findings corroborate the hypothesis that Plan B impairs the function of the corpus luteum, which would therefore deprive the fertilized egg of needed support during the several days after fertilization during which the embryo tries to implant into the endometrium (uterine lining).

In other words, if Plan B is given before ovulation, it may disrupt events that occur after fertilization, which ultimately impairs the ability of the embryo to successfully implant. An analogous example may be instructive. If someone tampers with a plane’s landing gear, the plane may still take off perfectly well, fly for a while, but when it is time to land, the plane will likely crash.

Plan B allows ovulation to occur, there are no barriers to fertilization and the egg is fertilized, but the newly formed embryo cannot implant into the endometrium because the corpus luteum has been damaged and cannot provide the embryo with the necessary environment.

Although there have been some studies examining the effect of Plan B on implantation which show that the drug does not impair the ability of embryos to implant upon an artificial endometrial construct or alter necessary endometrial receptivity molecules, all of these studies either gave the Plan B too late for it to show such changes or did not administer it to the women in vivo at all.[x] There are other studies that show that lowered progesterone levels lead to a whole host of molecular changes, which could interfere with implantation.[xi]
Next page: Catholic hospitals use Plan B?–>

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