What does the morning-after pill really do?

A big sore point in the Catholic Church's high-profile pushback against the Obama administration making most employers' health insurance plans provide copay-free birth control is the idea that Catholic hospitals, universities, and charities will be forced to support (directly or indirectly) "abortifacients" or "abortion-inducing drugs" — which refers to the morning-after pill, primarily Plan B. But anti-abortion advocates are wrong about what the morning-after pill does — as are abortion-rights proponents, the National Institutes of Health, the Mayo Clinic, and Plan B's label — according to a new examination of the research by The New York Times. So what, in fact, does the morning-after pill do? And can science neuter the controversy surrounding Plan B? Here's a look:

What's the common understanding of how the pills work?

Plan B, its generic version Next Choice, and a new competitor, Ella, are controversial, in large part, because people think they prevent a fertilized egg from implanting in the uterine wall, thus keeping it from surviving. That's "the moral equivalent of homicide," says Dr. Donna Harrison at the American Association of Pro-life Obstetricians and Gynecologists. This perception is supported by the FDA-mandated label on Plan B and its competitors, which says that preventing implantation is one possible way the drug averts pregnancy.

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According to The Times, what really happens?

The morning-after pill prevents fertilization from ever occurring, primarily by slowing the egg from entering the uterus until after the sperm die off, as long as five days after intercourse, according to leading scientists. Some pills also make it harder for the sperm to reach the egg by thickening the cervical mucus. But no studies have shown that the pill affects the egg once it is fertilized — the moment of conception, according to some religious precepts. It turns out that this politically charged fight over Plan B and abortion is "probably rooted in outdated or incorrect scientific guesses about how the pills work," says Pam Belluck at The New York Times.

How did people get the idea that the pill affects fertilized eggs?

Plan B was approved in 1999, and the language on the packaging about keeping a fertilized egg, or zygote, from implanting "reflects a period before the research confirmed what scientists already suspected," says Amanda Marcotte at Slate. And even as more and more studies show that the morning-after pills only slow ovulation, regulators are hesitant to change the label because of the "tendency in science not to rule out any possibilities until you have to." Then abortion politics entered the picture, and "unfortunately, the constant conflation of contraception and abortion has gone mainstream."

Does what's on the label really matter?

Both sides of the abortion debate "consider the wording on labels central because it summarizes scientific consensus and shapes what medical authorities say," says Belluck. So "theoretically, if a scientific consensus arises that says the pills do not prevent implantation," says Rebekah Kuschmider at Babble, that should remove any abortion-related stigma from Plan B, keeping it legal even if states adopt "fetal personhood" laws.

Will federal regulators change the label?

The FDA won't commit either way, but "it's possible," says Erin Gloria Ryan at Jezebel. "A growing chorus of scientists everywhere from the FDA to the Mayo Clinic to the National Institutes of Health seem amenable," and the makers of Plan B and Ella are lobbying for removing the language about post-fertizilation implanting.

Will these new findings end the debate around Plan B?

"Call me a cynic," but no, says Jezebel's Ryan. The anti-abortion side has too much invested in "the 'feticide' myth" to let it go that easily. And scientific facts certainly haven't prevented Republicans around the country from "pushing laws that would require doctors to lie to women about a supposed 'link' between abortion and breast cancer," says Dan Savage at The Stranger. Look, "I would be relieved" if the morning-after pill didn't terminate fertilized eggs, says Richard Doerflinger, anti-abortion director at the U.S. Conference of Catholic Bishops, but "so far what I see is an unresolved debate," and given the ethical problems with testing Plan B on women, "it's not only unresolved, but it may be unresolvable."

Sources: Babble, CNN, Jezebel, New York Times (2), Slate, The Stir, The Stranger