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charged debate over morning-after pills and abortion, a divisive issue in this election year, is probably rooted in outdated or incorrect scientific guesses about how the pills work.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, a medication prescribed for terminating pregnancies, destroys implanted embryos.