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 websites.

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 tha...