In a previous article, we covered what Plan B emergency contraception is and how it works, comparing it to the chemical abortion pills to which Plan B is often mistakenly conflated.
Still, people often want to know: Can Plan B can act as an abortifacient, i.e., can it induce an early abortion, or the ending of an established pregnancy? In this article, we’ll discuss Plan B’s three mechanisms of action for preventing pregnancy, and the reasons for the disagreements about whether or not it can sometimes act as an abortifacient.
Plan B has three possible mechanisms of action
According to its website, Plan B cannot cause an abortion because “It won’t affect an existing pregnancy,” and indeed, its primary mechanism of action is the prevention of ovulation.
But when you look at the actual back of the product box, you’ll read under “Other information” that Plan B could possibly work in two other ways as well: “this product works mainly by preventing ovulation (egg release). It may also prevent fertilization of a released egg (joining of sperm and egg) or attachment of a fertilized egg to the uterus (implantation).”
That third potential mechanism of action, prevention of implantation due to thinning or otherwise altering the endometrium (lining of the uterus), or by otherwise impacting the survivability of the newly conceived embryo, is why abortion opponents say Plan B could cause early abortions. Once fertilization has occurred, they point out, a genetically separate and distinct, albeit microscopic in size, human life (zygote) exists. Preventing successful implantation of the blastocyst (the next stage of fetal development after zygote and before the embryo stage) leads to the death of that new life, which abortion opponents say constitutes an early abortion. (Of note, the term “fertilized egg” employed on the back of the Plan B box is not a scientifically accurate term, because fertilization causes the existence of a genetically distinct entity with its own DNA, and is not just a further stage of development of an egg or a sperm.)
So, can Plan B cause an abortion or not?
To clear up the confusion around whether Plan B’s third mechanism of action constitutes an abortion or not, we have to look at changing medical terminology, going all the way back to 1965. In 1965 and then again in 1972, the American College of Obstetricians and Gynecologists (ACOG) redefined pregnancy from beginning at fertilization/conception to beginning at implantation, instead. (We won’t get into all of the details here surrounding why this definition change took place, and the historical context surrounding it, but you can read more about it here).
Suffice it to say that when it comes to whether or not Plan B can be considered abortifacient, the bone of contention between abortion opponents and abortion advocates centers on this discrepancy in definitions. Abortion opponents focus on Plan B’s effects on a newly conceived life, stating that because (according to the product packaging) Plan B has the potential to act in a way that renders a womb inhospitable so as to result in the death of that life, it should be considered an abortifacient (the same can also be said of combination birth control pills, which also function to keep the endometrial lining thin, in addition to other mechanisms of action, so that pregnancy will still be prevented should breakthrough ovulation occur). Abortion advocates state that because pregnancy is not clinically recognized until implantation, Plan B’s third mechanism of action (which is to prevent implantation), does not cause an abortion, regardless of whether the conception of a new human life has occurred.
Does Plan B work if ovulation has already occurred, and how often does it act to prevent implantation?
Regardless of whether one believes that disrupting a newly conceived human blastocyst’s ability to implant within the endometrial lining of the womb constitutes an abortion, ACOG and other mainstream reproductive health organizations say there is little evidence that Plan B could act in this manner.
ACOG believes the potential frequency of Plan B preventing implantation to be ‘Not very often’, and cites nine research studies to back up its claim. “Review of the evidence suggests that emergency contraception is unlikely to prevent implantation of a fertilized egg.” In these 2007 and 2011 studies, Plan B was wholly ineffective if given after the LH surge. Recall that the LH surge immediately precedes and signals impending ovulation [1][2].
Importantly, implantation occurs five or six days after fertilization, and fertilization can only take place within hours of ovulation. Recall that Plan B should be taken within 72 hours of “unprotected” intercourse to be effective, and ideally within 12 hours, according to the product’s manufacturers, and that it’s chiefly trying to prevent ovulation. The half life, or length of time that Plan B works in the body, is so short that health organizations warn women against trying to use it as normal birth control. Furthermore, some researchers and ACOG say it’s highly improbable that taking just one dose of synthetic hormone (Plan B’s active ingredient is the synthetic progestin levonorgestrel) would be enough to thin the endometrial lining to prevent implantation.
Obviously, Plan B cannot prevent ovulation if it has already occurred, and ACOG and other researchers say there’s no evidence that a single dose of Plan B can significantly alter the lining of the endometrium such that implantation (which, again, occurs five or six days after fertilization) could be prevented. While research to date has not consistently borne out the possibility of Plan B significantly altering the endometrial lining, more research is needed on its effects, especially for women who rely upon it as a regular form of birth control.
Arguments for Plan B’s potential abortifacient effect
Some researchers maintain that there are multiple credible scientific reasons based on the available research to believe that Plan B could be abortifacient, and the specifics of each of their claims (and responses to ACOG’s arguments) are here and here and here. Their main argument could best be summarized as: timing is everything. They note that the timing of when Plan B is taken in relation to ovulation could mean the difference between it preventing ovulation, having no impact at all, and impacting a newly conceived embryo. The crucial period for abortifacient action, they believe, is taking Plan B in the day or two immediately preceding ovulation, at which time some evidence suggests it is ineffective (already too late) at stopping ovulation. If ovulation occurs, fertilization may occur. Research further suggests that breakthrough ovulations occurring after Plan B consumption lead to luteal phase defects, characterized by shortened luteal phase length and decreased serum progesterone levels, which could logically lead to early abortion of a newly conceived embryo.
On the whole, research into emergency contraception over the last few decades has consistently been plagued by very small sample sizes and often by imprecise determinations of whether the research subjects were actually fertile at the time of Plan B ingestion. Additionally, much of it is 15-20 years old or older.
The bottom line: Plan B appears unlikely to act as an abortifacient
Research suggests that Plan B is ineffective if taken after ovulation has already occurred. For this reason, this specific type of contraception’s backup mechanism (as identified on the back of the box) of potentially preventing implantation–and conceivably causing the death of an early human life by creating an inhospitable environment in the womb–appears to be biologically improbable. (In fact, regular hormonal birth control, which functions to keep the uterine lining consistently thin, may be more likely than Plan B to act in this manner in the event of breakthrough ovulation–more on that, here.)
While Plan B may be unlikely to act as an abortifacient, its effects on a woman’s body are not necessarily benign. Containing as it does the synthetic progestin levonorgestrel, with the known side effects of breast tenderness, dizziness, irregular bleeding and/or menstrual changes, headache, fatigue, abdominal pain, and nausea, some women may find taking Plan B an unpleasant business. Significant side effects make sense given that just one dose of Plan B contains 33%-50% of a total month’s supply of levonorgestrel as found in levonorgestrel-containing birth control pills like Aviane, Lessina, and Levora. And anecdotal stories suggest that, at least for some women, Plan B’s effects may last for multiple cycles after taking it.
Rather than being encouraged to take (and/or stock-up on) emergency contraception, women would be better served by learning the empowering information about their cyclic fertility that comes with being trained in a method of fertility awareness or natural family planning. Women also deserve to know all of the evidence-based information about the side effects and risks of various birth control methods, including Plan B, so they can make truly informed family planning decisions for themselves.
Editor’s Note: In December of 2022, the FDA changed the Plan B product label to remove language implying that a backup mechanism of action could be abortifacient via thinning the lining of the endometrium or otherwise making it inhospitable to a newly conceived embryo. The label now reads “Levonorgestrel emergency contraception (LNG-EC) is effective in preventing pregnancy when taken within 3 days after unprotected intercourse. LNG-EC does not terminate an established pregnancy and does not affect a continuing pregnancy.” All of the research cited for the label change is more than 10 years old, which does not invalidate its inclusion but does seem to reflect a change in semantics rather than incorporation of new data that meaningfully changes what’s long been known about the medication’s mechanisms of action.
This article was last updated on January 31, 2023.
References:
[1] Novikova, Natalia et al. “Effectiveness of levonorgestrel emergency contraception given before or after ovulation–a pilot study.” Contraception vol. 75,no. 2 (2007): pp. 112-8. doi:10.1016/j.contraception.2006.08.015 [2] Noé, Gabriela et al. “Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation.” Contraception vol. 84,no. 5 (2011): pp. 486-92. doi:10.1016/j.contraception.2011.03.006
