Morning-After Pills Don't Block Implantation, Science Suggests Labels inside every box of morning-after pills, drugs widely used to prevent pregnancy after sex, say they may work by blocking fertilized eggs from implanting in a woman's uterus. Respected medical authorities, including the National Institutes of Health and the Mayo Clinic, have said the same thing on their Web sites. Such descriptions have become kindling in the fiery debate over abortion and contraception. Based on the belief that a fertilized egg is a person, some religious groups and conservative politicians say disrupting a fertilized egg's ability to attach to the uterus is abortion, "the moral equivalent of homicide," as Dr. Donna Harrison, who directs research for the American Association of Pro-life Obstetricians and Gynecologists, put it. Mitt Romney recently called emergency contraceptives "abortive pills." And two former Republican presidential candidates, Newt Gingrich and Rick Santorum, have made similar statements. But an examination by The New York Times has found that, the federally approved labels and medical Web sites do not reflect what the science shows. Studies have not established that emergency contraceptive pills prevent fertilized eggs from implanting in the womb, leading scientists say. Rather, the pills delay ovulation, the release of eggs from ovaries that occurs before eggs are fertilized, and some pills also thicken cervical mucus so sperm have trouble swimming. It turns out that the politically charged debate over morning-after pills and abortion, a divisive issue in this election year, is probably rooted in outdated or incorrect scientific guesses about how the pills work. Because they block creation of fertilized eggs, they would not meet abortion opponents" definition of abortion-inducing drugs. In contrast, RU-486 is an abortion pill because it destroys implanted embryos, terminating pregnancies. The implantation idea stems from the Food and Drug Administration's decision during the drug-approval process to mention that possibility on the label - despite lack of scientific proof and objections by the manufacturer of Plan B, the pill on the market the longest. Leading scientists say studies since then provide strong evidence that Plan B does not prevent implantation, and no proof that a newer type of pill, Ella, does. Some abortion opponents said they remain unconvinced. After The Times asked about this issue, A.D.A.M., the firm that writes medical entries for the National Institutes of Health Web site, deleted passages suggesting emergency contraceptives could disrupt implantation. The New York Times, which uses A.D.A.M."s content on its health Web page, updated its site. At the Mayo Clinic, Dr. Roger W. Harms, the Web site's medical editor in chief, said "we are chomping at the bit" to revise the entry if the Food and Drug Administration changes labels or other agencies make official pronouncements. "These medications are there to prevent or delay ovulation," said Dr. Petra M. Casey, an obstetrician-gynecologist at Mayo. They don't act after fertilization. The F.D.A. declined to discuss decisions about the implantation effect or to say whether it would consider revising labels. But Erica Jefferson, an F.D.A. spokeswoman, acknowledged: "The emerging data on Plan B suggest that it does not inhibit implantation. Less is known about Ella. However, some data suggest it also does not inhibit implantation. Scientists say the pills work up to five days after sex, primarily stalling an egg's release until sperm can no longer fertilize it. Although many people think sperm and egg unite immediately after sex, sperm need time to position themselves. Controversy over emergency contraception is figuring into the presidential race and debates over the Obama administration's health care law. Some abortion opponents and religious groups are fighting the law because it requires insurers, including those for employees of Roman Catholic institutions that oppose birth control, to cover contraceptives, including morning-after pills. While some object to contraception generally, others focus on birth control methods that they believe cause abortions. And some proponents of "personhood" initiatives, proposals being put forward in several states to define fertilized eggs as people, say the initiatives would bar the pills if they work after eggs are fertilized. Doctors also say some patients who are not active on abortion issues want to resolve ethical questions about whether the pills affect a fertilized egg. A Growing Market While scientific and political disputes over emergency contraception may persist even if labels change, both sides consider the wording on labels central because it summarizes scientific consensus and shapes what medical authorities say."F.D.A. labeling is important to us, it's very important," said Jeanne Monahan, director of the Center for Human Dignity at the Family Research Council, a conservative group. Diana Blithe, a biochemist who oversees contraception research for the National Institutes of Health, the federal agency for medical research, said implantation should be removed from labels. "As a scientist, I would definitely take it off of emergency contraception," she said. Ninety-nine percent of the emergency contraception market in the United States consists of Plan B (approved in 1999 and now sold as Plan B One-Step) and its generic versions, Next Choice and levonorgestrel tablets. They are available without prescription for ages 17 and older. The other pill, Ella, became available by prescription only in the United States in late 2010. Emergency contraceptive use has steadily increased, with about 12 million packages sold last year, according to IMS Health and SymphonyIRI Group, health information and market research companies. European medical authorities have not placed implantation on Ella's label, and after an additional two years of scrutiny, Ella was approved for sale in overwhelmingly Catholic Italy, where laws would have barred it if it could be considered to induce abortion, said Erin Gainer, chief executive of Ella's manufacturer, Paris-based HRA Pharma. Some abortion opponents said that while emergency contraceptives" primary function may be delaying ovulation, they doubted that scientists could exclude the possibility of implantation effects.