Questions About Morning After Pill


I am the Nurse Manager at a California Pregnancy Counseling Center and I used to be a FertilityCare Practitioner with the Creighton Model. I believe I have contacted you before regarding other topics (like abortion pill reversals or contraception information, etc.) but since these questions will sound very suspicious, I thought I should mention these things ahead of time, since I am trying to give our clients as much truthful information about the medications as possible.

First, I know that the morning after pills (Plan B) are just large doses of synthetic progesterone I’ve read they are anywhere from 4-40 times the dose in a regular birth control pill.) My question is: if a woman has just ovulated, will this large dose of progesterone really have any negative effect on the lining of the uterus? As I recall from the Creighton Model, some women are given progesterone supplements (bioidentical, usually) in their post-peak phase if they’re at high risk for miscarriage, and they are given the progesterone supplements throughout their first trimester if necessary. So is the morning after pill really an abortifacient? It seems to me that the extra progesterone (although it might be unnecessary) would just ensure the lining of the uterus is in place if conception has occurred but implantation has not yet occurred. I also know the morning after pill doesn’t work well to prevent pregnancy — perhaps it doesn’t really work at all, since most women have no idea where they are in their cycle and wouldn’t know if they’re fertile or not. I can’t tell you how many women I’ve had at the clinic who’ve said they took the morning after pill and they still got pregnant!

My second question is along the same line of thinking: the abortion pill RU-486 is a progesterone receptor blocker, right? So if a woman regrets taking the RU-486 and wants to reverse it (assuming she has not taken the Cytotec yet,) could she theoretically take the morning after pill to have the synthetic progesterone compete for the receptors that the RU-486 blocks? (Not that I would EVER suggest or advocate something like that! Both drugs have so many other horrible side effects, and abortion pill reversal needs to be done under a doctor’s supervision, plus the woman would need to be checked to see if the baby is still viable.)

Thank you so much for your time in considering these questions.


Last Updated: March 11, 2024

Dear Christie,

We really are not sure if the progestin in Plan B thins the lining of the uterus (endometrium). The Plan B package insert lists thinning of the endometrium as a possible mechanism of action. It may be that the timing of its ingestion in relation to a woman’ cycle will determine the effects.

From package insert: CLINICAL PHARMACOLOGY Emergency contraceptives are not effective if the woman is already pregnant. Plan B® is believed to act as an emergency contraceptive principally by preventing ovulation or fertilization (by altering tubal transport of sperm and/or ova). In addition, it may inhibit implantation (by altering the endometrium). It is not effective once the process of implantation has begun.

We do know that women on combination birth control pills do have endometrial thinning.

I would not use the morning after pill to block mifepristone due to androgenic effects of artificial progestins. In health,

George Delgado, M.D

Answered By:

George Delgado, M.D., F.A.A.F.P.
George Delgado, MD, President and Founder of Steno Institute and Medical Director of Culture of Life Family Services (COLFS), San Diego, is a Professional Member / Supporter of CANFP. Board certified in both family medicine and hospice and palliative medicine, Dr. Delgado received his medical degree from the University of California, Davis, and completed his residency at Santa Monica Hospital/UCLA. Dr. Delgado is a Natural Family Planning Medical Consultant, trained in NaProTechnology. He and wife have four children and seven grandchildren.

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