I spent the evening researching progesterone in early pregnancy and the FDA debate concerning the risk after ovulation and/or fertilization to any embryo that might be present when a woman or girl takes Plan B, which contains the progesterone, levonorgestrel.
More evidence that the Plan B formulation should not be a risk to the early pregnancy (after fertilization, before or after implantation) includes:
1. The statement by Dr. Linda C. Guidice, MD, PhD, then-Chair of the Advisory Committee on Reproductive Health Drugs (approximately page 202 of 314 of the document) to the joint meeting of FDA advisory committees concerning changing the status of Plan B to Over the counter:
DR. GIUDICE: Actually I have two comments. One is that a five-day window can be interpreted with the sperm being in the reproductive tract for 72 to 96 hours with a very late ovulation and with an effect of the levonorgestrel on a decreased release of the sperm in the cervical mucous or in the crypts of the fallopian tubes.
Secondly, for fertility therapy we commonly begin progesterone administration on post ovulatory day 2, and for infertility therapy with embryo transfer, we commonly begin supplemental progestin or progesterone one day before embryo transfer.
So I just want to make it very clear that administration of progesterone clinically early and periovulatory has no significant impact upon implantation rates.
2. Statement in the review of evidence (on p. 13)in the Government Accounting Office investigation into the circumstances surrounding the refusal to change the status, with footnotes:
ECPs have not been shown to cause a postfertilization event—a change in the uterus that could interfere with implantation of a fertilized egg.29 Some researchers argue that an interference with the implantation of a fertilized egg is unlikely to happen because progestins, whether natural or synthetic, help to sustain pregnancy.30 In addition, there is no evidence that one burst of levonorgestrel without estrogen can prevent implantation. However, researchers have concluded that the possibility of a postfertilization event cannot be ruled out, noting that it would be unethical and logistically difficult to conduct the necessary research.31 ECPs, including Plan B, do not interfere with an established pregnancy.
29Implantation is the embedding of the fertilized egg in the uterus six or seven days after fertilization. See A.L. Muller and others, “Postcoital Treatment with Levonorgestrel Does Not Disrupt Postfertilization Events in the Rat,” Contraception, vol. 67 (2003): 415-419.
30Horacio B. Croxatto, Maria E. Ortiz, and Andres L. Muller, “Mechanisms of Action of Emergency Contraception,” Steroids, vol. 68 (2003):1095-1098.
31It has not been possible to identify groups of women who had taken ECPs after fertilization so as to assess their effect on the establishment of a pregnancy. Therefore, there is no direct evidence, either for or against, the hypothesis that ECPs prevent pregnancy by affecting postfertilization events. See Croxatto, Ortiz, and Muller, “Mechanisms of Action of Emergency Contraception,” 1096.
3. Progesterone levels in early pregnancy, with graphs showing increase in progesteron after ovulation
4. Use of Progesterone in therapy for treatment of infertility and recurrent early pregnancy loss
Edited for formatting 5/1/13 – BBN