On August 24, I wrote a piece about Plan B, which the FDA had just approved. Afterward, someone left this comment:

How do they (scientists) not know whether or not Plan B sheds or doesn’t shed the uterine wall? That seems like it would be simple to find out. If Plan B did in fact do this…would this change some of your opinions?

In my original entry, I quoted William Cutrer, M.D. He was a graduate of Dallas Theological Seminary, had been an ob-gyn for about two decades, and had been on the front lines of the pro-life movement offering free obstetrical care to thousands of women at risk for abortion. He taught at The Southern Baptist Theological Seminary in Louisville, Kentucky until the time of his death in 2013. I asked him to respond to this reader’s question because it’s so important that we understand the medicine. Here is what he had to say:

Before we can talk about the ethics, we have to be on the same page about how Plan B works. That requires an understanding of some of the events in a woman’s menstrual cycle required for an embryo to implant. This is a complex synchrony, and many factors can alter the process. But we begin with some basics:

1. There can be no pregnancy without ovulation. No egg = no embryo.
2. For an egg to mature enough to ovulate, there must be the gradual increasing production of estrogen in the ovary.
3. The estrogen has two effects—it matures the egg and it grows the uterine lining.
4. Estrogen won’t happen without messenger hormone (FSH) from the pituitary gland being secreted appropriately for several days before ovulation.
5. Progesterone, named for its ability to support (pro) a pregnancy (gestation), begins to be secreted by the part of the ovary that ovulated. Ovulation followed by progesterone production can’t happen unless the pituitary gland releases a burst of another messenger hormone (Luteinizing hormone, or “LH”).
6. The estrogen grows and “thickens” the lining of the uterus during the run-up to ovulation, and the progesterone “sweetens” this thickened lining by increasing the glycogen (sugar stores) in those cells.
7. Once ovulation occurs, fertilization takes place in the fallopian tube. The growing embryo divides several times over the course of about three days before arriving in the uterus. It “floats” there for a few more days before beginning the process of implantation.
8. From the time of ovulation to implantation, progesterone has had a number of days to prepare the estrogen-thickened lining for the arrival of the embryo. In the treatment of infertility patients, including those at risk for miscarriage, we occasionally add natural progesterone to make sure the lining of the uterus is optimally favorable for an implantation.

Now, Plan B = progesterone. That’s all it is—just progesterone. Unlike combination oral contraceptive pills, which contain both estrogen and progesterone, Plan B contains only progesterone in a synthetic form.

One could anticipate that, if taken before the LH surge triggering ovulation, Plan B/progesterone would block the release of an egg, thus preventing fertilization. Once ovulation has occurred, it is doubtful that progesterone alone would a ) block the FSH thereby inhibiting secretion of estrogen (thus thinning the lining or, more accurately, preventing the thickening) or b) be enough, with only the two-dose, when withdrawn, to cause the uterus’s lining to slough off, as happens in a menstrual period.

The effect of Plan B on the ability of the embryo to travel as it needs to down the tube, on cervical mucus, and other potential factors has yet to be evaluated.

If Plan B were to be strong enough to cause “shedding of the endometrium,” we would see withdrawal bleeding a few days after taking the pills. But, if fertilization had already occurred, the ovary would still be making more progesterone in all likelihood, which is why there’s a clear “warning” in the packaging that comes with Plan B that says it does not work after implantation has begun.

So, the only ethical question remaining is this: Does Plan B have any significant effect as an abortifacient when taken during the time between ovulation and implantation? That remains a much tougher window to evaluate, particularly since the product is relatively new. Studies seem to suggest that it will prevent ovulation, but won’t prevent implantation of an embryo.

Consider recent research by members of the Population Council’s International Committee for Contraception Research (ICCR) and other scientists. It shows that “the most popular method of emergency contraception appears to work by interfering with ovulation, thus preventing fertilization, and not by disrupting events that occur after fertilization.” See

My personal feeling about Plan B is this: it’s acceptable in cases of rape/incest where ultrasound demonstrated that ovulation has not yet occurred. In such cases, Plan B could block the LH surge, preventing ovulation, and avoiding an unwanted pregnancy—not by destroying an existing embryo, but by blocking ovulation.

Otherwise, as sold over the counter, without any medical advice or evaluation, I would oppose its use, and do oppose its sale without prescription. My opinion is that this drug will be misused in many ways by people who don’t understand its mechanism of action, and it may result in more unwanted pregnancies and possible surgical abortions because people will use Plan B thinking it prevents every pregnancy regardless of when in the cycle the pills are taken.