Anybody who follows reproductive rights news (and if you ever have sex, or plan to, you should) knows that Plan B, the much-discussed morning after pill, is the first non-barrier method of birth control that you can get over the counter. 'Course, you still need an ID to prove you're over 18—or you can get a legal adult friend to come with you—and the price varies ridiculously, from $10 at, say the Hennepin County Medical Center in Minneapolis to $54 at my local CVS. It doesn't prevent STDs, it's slightly less reliable than the regular pill, it's not a substitute for other forms of birth control, yadda yadda. But. It's a good thing regardless, now that we know we can get it.

What most people do not know, however, is how it actually works. The great Christian War on Sex (I'm copyrighting that, by the way) propagandists have successfully convinced most people that Plan B is an abortifacient, because the manufacturer's drug information says that it "may" prevent a fertilized ovum from attaching to the uterine wall.

So let me explain a few things. First, how the drug actually works. Plan B consists of two pills that contain a synthetic form of progesterone, one of the two main female hormones (the other is estrogen) that occur naturally and are used in birth control pills. Men, by the way, have progesterone too, though in lower doses. When you take Plan B—two pills twelve hours apart—you get a very large dose of progesterone that lasts for a day or two.

When a woman is pregnant, her body produces a lot of progesterone, which keeps her from ovulating—thereby preventing her from getting pregnant a second time and trying to carry them both, which would be a disaster. So progesterone prevents ovulation. Which, if you're a woman who doesn't want to get pregnant, is a pretty useful thing for it to do.

So let's say for some reason a woman who isn't on birth control finds herself with sperm floating around inside her: a condom broke, she was raped, she and her partner were sloppy and overly enthusiastic, whatever. If she's already ovulated within the last 24 hours, all she can do is cross her fingers: you can't stop something that's already happened. But if she hasn't, and is unlucky enough that she's just about to, she can take a big dose of progesterone that will stop her from popping an egg long enough for the sperm to die, and no pregnancy. Hurrah!

But, you ask, what if she's already ovulated, and the sperm luck out, and there's a zygote floating around but it hasn't yet stuck itself to the wall of the uterus? Would Plan B work then? And if it does, isn't that technically a kind of abortion?

Nope, and nope. If you've ovulated, the zygote either is or isn't going to be lucky enough to find a resting place. A lot of them don't, which is why pregnancy starts once the zygote takes root in the uterine lining. If you're not pregnant, you can't abort.

Okay, but. What if there's a zygote, and you personally see fertilization as the beginning point of life, even if it happens before pregnancy actually starts? If Plan B prevents implantation—and their own literature says they "may" do that—then that feels iffy to me.

This is the second thing that needs explaining. There is no evidence that Plan B prevents implantation. That's not a known effect of progesterone. But because science can't prove a negative—you can't prove that something doesn't happen, because if it doesn't happen, there is, by definition, no evidence—the drug company puts that little "may" in there. All "may" means is "even though we've never seen it happen, we can't prove that it doesn't." It's kind of like saying, "well, it might cause your skin to turn blue," or "it might cause you to grow a third arm," although they don't bother to say *that*, because it's not a possibility that would occur to anyone. But there is no evidence—none, zilch, nada—that Plan B prevents implantation. All it does is help you control the timing of ovulation; sort of the women's equivalent of guys thinking about baseball or their mother in order to keep from coming. For a guy, orgasm and ejaculation are the same event: for a woman, orgasm and ovulation aren't. If you're a guy, you can usually prevent the latter by controlling the former; if you're a woman, all you have to do is take a pill.


I got a lot of the details about this stuff from PZ Myers at Pharyngula: see Why the Wingnuts Hate Plan B and Plan B, Again.


Bitch_PhD writes the Bitch, Ph.D blog, may or may not still be a professor, and paid $45 for Plan B last month, but is happy to say she finally renewed her regular birth control prescription yesterday.