Fifth in a series.
In this post, I spoke of three evidences that have been presented to support a post-fertilization mechanism of action from Plan B EC. I promised to challenge each one, and this was #2:
2. Since EC has the same types of hormones found in regular oral contraceptives, and there is evidence that regular OCs can have a post-fertilization event, then it stands to reason that EC would also have a post-fertilization mechanism of action.
I believe this point is the easiest one to refute, and my research in this area has turned up some surprises.
First, this point implies that there is a known post-fertilization effect from regular OCs. There is no consensus on that issue, and there is no direct evidence that OCs cause a "hostile endometrium." However, even if you believe that regular OCs do cause abortions, that does not indicate that Plan B EC does work via a post-fertilization event. This was a surprising aspect of this research: if Plan B acts after fertilization, the evidence states argues that it must do so by a mechanism that is different than regular OCs. I will show why.
Most who believe that regular OCs can act as an abortifacient use what I call the "hostile endometrium" theory". The theory is that continued use of OCs create a thinner, less vascular endometrium that would be less likely to accept an embryo attempting to implant. Randy Alcorn, who has written extensively on this topic, explains it this way:
When the Pill thins the endometrium, it seems self-evident a zygote attempting to implant has a smaller likelihood of survival. A woman taking the Pill puts any conceived child at greater risk of being aborted than if the Pill were not being taken...
First, after a woman stops taking the Pill, it usually takes several cycles for her menstrual flow to increase to the volume of women who are not on the Pill. This suggests to most objective researchers that the endometrium is slow to recover from its Pill-induced thinning
Now if the effect of regular OCs take months to for a woman's body to "undo", then how does Plan B EC create a hostile environment in a matter of hours? It seems that if EC works via a post-fertilization event, it must use some different mechanism than regular OCs, which appears to be based on a chronic thinning of the endometrium.
However, I did not have to depend on the argument of the last paragraph. There is actually experimental evidence that shows that Plan B taken after ovulation does not have any significant change in the morphology of the endometrium. Take a look at the abstract here and the full article here that presents this evidence. (Contraception. 2001 Oct;64(4):227-34.) They performed endometrial biopsies on women who took Plan B EC and control groups and found no difference in the morphology between the two. The paper states:
No significant changes were observed between treated and control specimens in any of the studied parameters. No significant differences among groups were observed. Of particular importance was the finding that the predecidual changes as evaluated by the presence of prominent spiral arteries, which are considered crucial for implantation , were not altered by LNG.
Remember, the proposed mechanism of regular OCs is to create a significantly thinner endometrium. This does not occur when a woman takes Plan B.
Although the medications are the same, if Plan B acts to cause abortions, it seems to have to do so via a different mechanism than does regular OCs. Instead of being evidence for a post-fertilization mechanism of action, this actually gives support to the theory that Plan B does not act after ovulation.