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I don't believe that there is any evidence that the Plan B protocol prevents implantation. For some reason, both sides of the abortion issue continue to quote 30-old information that never had any supporting evidence.

However, newer evidence has been validated by more than one investigator and was reported to the advisory committee that reviewed the protocol last year for the FDA.

The evidence is not all in, but I am pretty much convinced that the protocols only work before ovulation and any effect after ovulation would serve to slow the sperm so that it's less likely to get to the oocyte in the fallopian tube, where fertilization takes place and to actually *encourage* implantation if there is fertilization.

This latter makes since when we remember that one of the reasons for miscarriage is low progesterone and fertility docs often give progesterone to women early in pregnancy.

The most significant are the studies from Brazil by Croxatto and his group (H.B. Croxatto et al. / Contraception 70 (2004) 442–450)," which was a blind study, cycling women who were otherwise unable to get pregnant (sterilized or with - what I consider unethical and potentially abortifacient - IUD's) through 3 courses - placebo, and two forms of progesterone-only pills. The researchers followed the women with serial ultrasound and hormonal blood essays.

There is a study by Durand, et.al. (M. Durand et al. / Contraception 71 (2005) 451– 457), from 2001 which tested surgically sterilized women given 2 doses of Levonorgestrel, 12 hours apart. These women were studied by serial ultrasounds and women who ovulated also underwent endometrial biopsy.

There was no difference in their uterine lining function or anatomy although there was a difference in the expression of glycodelin-A. This protein prevents binding of the sperm to the zona pellucida of the oocyte and so, prevents fertilization. (There is some speculation that the protein acts to help implantation, too.)

In the meantime, the Catholic hospitals do sometimes use a protocol that involves the use of a test for lutein hormone, indicating whether or not a woman has ovulated. They only prescribe the protocols if she is not pregnant (and so, she was pregnant before the unprotected sex in question) and has not yet ovulated (so there is no oocyte to fertilize).
Even this use is controversial. And it's still ontroversial.

Croxattoa, et al., Pituitary–ovarian function following the standard levonorgestrel emergency contraceptive dose or a single 0.75-mg dose given on the days preceding ovulation Contraception 70 (2004) 442–450

Durand, et. al., On the mechanisms of action of short-term levonorgestrel administration in emergency contraception. Contraception 64 (2001) 227–234

Seppala, et. al., Glycodelin: A Major Lipocalin Protein of the Reproductive Axis with Diverse Actions in Cell Recognition and Differentiation Endocrine Reviews 23 (4): 401-430 Copyright Â© 2002 by The Endocrine Society.

It sounds so right, what you are saying. Indeed, reproductive endocrinologists give large amounts of progesterone to their patients trying to conceive.

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.