I know this is tough for people who oppose basic sex ed to understand, but you do not actually get pregnant the second a dude ejaculates inside of you. The sperm have to swim! The egg has to be released! The two must meet! There are many factors. And the morning-after pill interferes with some of them, keeping the egg from getting fertilized in the first place:
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charged debate over morning-after pills and abortion, a divisive issue in this election year, is probably rooted in outdated or incorrect scientific guesses about how the pills work.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.
None of this is news to the pro-choicers who have actually read studies on the morning after pill and understand basic biology. But it’s nice to see it all spelled out.