Once again, the evidence is good that the Plan B progesterone-only morning after pills only work before
ovulation during a narrow five-day range and they do not prevent implantation after fertilization - there's a good chance that the progesterone increases the implantation rate of any embryos begun after ovulation.
Just look at the information from this natural planning information website:
http://www.woomb.org/bom/science/physiology.html
See Figure 1
In fact, we give progesterone to protect early pregnancies. In natural cycles, progesterone increases after ovulation.
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.)
That settles it for me.
Thank you. I'm pro-"Plan B".