How Plan B Works Before and After Ovulation

Research Findings vs. FDA Labeling

Many women may choose to use Plan B One-Step (or its generic forms, such as My Way, Take Action, and Next Choice One Dose) as emergency contraception to prevent pregnancy after birth control failure or unprotected intercourse.

Plan B Pill box
Justin Sullivan / Getty Images

Even though emergency contraception is safe and effective, the use of Plan B continues to generate controversy. Much of this debate stems from how Plan B works—specifically with respect to whether or not Plan B prevents fertilized eggs from implantation.

It also seems that a major factor contributing to the confusion about how Plan B works is that the FDA labeling for this product says one thing, yet research and data do not confirm the FDA’s labels.

What the FDA Labeling Says

According to product labels, Plan B works mainly by preventing/delaying ovulation or by avoiding fertilization (by altering the movement of sperm and/or egg).

The debate arises from the next part of the label which, under "mechanism of action," reads: “In addition, it [Plan B] may inhibit implantation (by altering the endometrium).” However, newer research findings have been inconsistent with that finding.

The medical community is divided on the definition of pregnancy. The legal definition of pregnancy "encompasses the period of time from implantation until delivery." Yet those who hold personal viewpoints that pregnancy starts at fertilization believe that Plan B is akin to abortion since its use can prevent implantation of a fertilized egg.

That being said, research does not support this theory about how Plan B works. In fact, studies show that when taken after ovulation, Plan B does not decrease the rate of pregnancy.

What Research Shows

The majority of the research reveals that using Plan B does not cause any changes in the lining of the uterus (endometrium). Because Plan B does not have any effect on the endometrium, researchers have concluded that this emergency contraceptive cannot prevent implantation of a fertilized egg.

Many researchers have further explained the notion that Plan B does not stop implantation from occurring is probably the reason that it's not 100 percent effective at preventing an unintended pregnancy, as well as why Plan B is less effective the longer you wait to use it.

Research shows Plan B does not cause any changes in the lining of the uterus and therefore would not prevent implantation of a fertilized egg.

Implantation Controversy

From the very beginning of Plan B’s approval process, its manufacturer requested that the FDA not include implantation as one of the ways Plan B works on its label.

It appears, though, that during the approval process, most of the research on Plan B investigated whether its active ingredient, the progestin levonorgestrel, could safely and effectively prevent pregnancy. These studies did not explore how Plan B works.

So, the FDA may have decided to include implantation on Plan B’s labeling because this mechanism seems to be one of the ways that birth control pills work (those that contain levonorgestrel as well as brands that use a different progestin)—by changing the lining of the uterus. So the FDA may have decided that because the pill may do this, so can Plan B.

But there are two things to keep in mind:

  • Even if the pill does alter the endometrium, the changes it causes have not been proven to hamper the implantation of a fertilized egg.
  • Yet more importantly, with birth control pills, you are building up progestin doses because you take these pills every day. Research shows that the single levonorgestrel dose in Plan B doesn’t have the strength or time to cause any changes to the lining of the uterus.

Yet those involved in the Plan B approval process report that the FDA decided to include, on the product label, the possibility that Plan B works " interfering with a number of physiological processes." The FDA then expanded upon this statement by including a list of all the possible ways that Plan B works.

Even though there was no scientific proof that Plan B works by preventing eggs from implanting, this mechanism of action was still included by the FDA on the drug’s label.


The FDA is not considering a label revision at this time, Erica Jefferson, an F.D.A. spokeswoman, acknowledged, "The emerging data on Plan B suggest that it does not inhibit implantation." Research does make clear that Plan B will not interrupt an existing pregnancy, and works primarily by preventing ovulation. 

Because the research overwhelmingly shows that Plan B does not prevent implantation of a fertilized egg, the research community (and many in the medical community) feel that such language about implantation should be removed from Plan B labeling.

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