Yet it is estrogen that builds the lining, and the doctors fill up their patients with it FIRST before adding the progesterone. Perhaps progesterone's effects STOP the further growth of the previously grown uterine lining.
I stand corrected on some of the posts I wrote. What the morning after pill probably does is:
A) If early in the cycle, it has an effect on unripened follicles, possibly delaying them ripening to eggs
B) If later in the follicular phase, it stops the growth of the uterine lining at an early stage if follicles have already started to ripen .
Obviously a uterine lining of only a few millimeters will not sustain the embryo if conception does occur.
And some might be taking the post-sex pill late in their cycle, after ovulation (they would of course never have gotten pregnant).
The minipill used as daily birth control is progestin only, and it has a high rate of failure. No doubt the morning after pill might, too, but people don't conceive every month they try to, anyway. One episode of sex, even during a fertile time, doesn't automatically result in pregnancy. So the morning after pill's got that going for it too.
Durand, confirmed the results from earlier studies - there is no change in the thickness of the uterine lining from a single dose of levonorgesrel,
"These results also correlate with the presence of normal histopathological features in endometrial biopsies taken during the implantation window in women from Groups B, C, and D. Indeed, in this study, the process of transformation of endometrium into decidua, as a consequence of endometrial cell differentiation independently of conception occurred normally in women receiving LNG at the time or after the occurrence of LH surge. In this regard, the existence of edematous changes along with development of prominent spiral arteries in LNG-exposed tissues strongly suggest the apparent preservation of endometrial structures thought to be associated with implantation capabilities [45,46]. A successful implantation requires an adequate synchronization between embryo and endometrium development for invasion of spiral arteries by trophoblast cells [47]. These results suggestthat postovulatory contraceptive efficacy of LNG may not involve alterations in the mechanisms associated with endometrial receptivity. A similar conclusion was reached by Raymond et al. [48], where no substantial evidence for the contraceptive effect of the Yuzpe regimen."
Also, the lining in the fallopian tubes is very thin, yet it's capable of sustaining the embryo. In fact, embryos implant in the visceral tissues of the abdomen, at times.
The mini pill is dangerous because it doesn't block ovulation as effectively as the combination oral contraceptives and because, when there is ovulation, the movement of the oocyte and the sperm in the fallopian tube are slowed for a few hours after each dose, resulting in 10 times the normal rate of tubal pregnancies.
However, it doesn't appear that a single dose of progesterone is significant for increasing the risk of tubal pregnancy, probably because of the short window of time that it works to slow cilia motility.