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.

But an examination by The New York Times has found that the federally approved labels and medical websites 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.