and since we know that the progestin in the pill is a synthetic form of progesterone which does not act like natural progesterone to prepare the uterus for implantation, it ends up working like a slower, low dose mifepristone. Yes, mifepristone has double the binding site affinity as normal progesterone and progestins and therefore it works much more harshly and quickly, but they are both "progesterone Narcan" as far as filling progesterone receptors but yet blocking the function of the woman's natural progesterone to prepare the uterus for implantation. A woman's own progesterone works best to prepare the uterus and maintain a pregnancy, that's why so many doctors and others have switched to bioidentical progesterone for treating infertility/miscarriage/menopause.
The assertion here is that LNG, the progestin found in Plan B, works as a competitive antagonist for progesterone receptors like Narcan works for opioid receptors. She doesn't provide evidence for this, and I believe there are at least three reasons why it is very unlikely.
First, the amount of progesterone secreted by the corpus luteum after ovulation is far greater than that exogenous progestin absorbed by taking Plan B. The notion that the small amount of oral progestin that effects the endometrium would overcome the huge surge of natural progesterone secreted in the luteal phase of the cycle is highly unlikely and would need evidence to be compelling.
Second, if LNG acts as a "weak" mifipristone (RU-486), then it should also have the effect of exhausting the progesterone receptors after inplantation has occurred. In all studies, LNG has had no effect on pregnancies after implantation. Why not?
Third, and I believe most compelling, is that other progestins are used in order to treat luteal insufficiency. When a women does not produce enough progesterone during the luteal phase of her cycle it has been shown that progestins have had a positive effect on preventing spontaneous miscarriage. It is true that bioidentical progesterone has been shown to be the best supplement for the insufficient natural progesterone in these patients (not surprisingly), but synthetic progestins did not have a negative effect, but a positive one.
In short, the assertion that the LNG in Plan B acts as a competitive antagonist is not only not supported in the literature, there is a number of reasons see that the evidence shows it is highly unlikely.
I'll respond to Marie's other comments in a future post.