Accidents happen. In the case of intercourse without any birth control on board, there are a few options to prevent pregnancy from occurring. To be clear, emergency contraception doesn’t cause an abortion of an existing implanted pregnancy. All of these methods work by preventing ovulation, fertilization, or implantation. 

The Morning-After Pill, also called Plan B, is one or two doses of hormones taken after unprotected intercourse to prevent pregnancy. The most common Morning-After Pills are progesterone alone, but an estrogen-progesterone combination also works. You need to take it within three days of the event, but the sooner the better. If taken before ovulation, progesterone can prevent the release of an egg from the ovary. If you’ll remember, a hormone surge from the brain triggers ovulation, but progesterone can prevent this surge and thereby prevent ovulation. No egg released means no pregnancy. However, this only works if taken earlier than the day of ovulation. Once the brain hormones surge, an egg is likely to be released whether you take progesterone or not. 

If an egg is already released or fertilized, such a short dose of hormones won’t interfere with implantation. Nor will those hormones interfere with an embryo that has already implanted. 

Side effects of progesterone-only pills are generally mild: headache, abdominal pain, nausea, or menstrual cycle irregularity. However, the side effects of the combined estrogen-progesterone pills are awful: nausea and vomiting are common. Most women who get prescribed the combination pill should get a prescription for an antinauseant as well. 

Overall, the Morning-After Pill works most of the time, with a failure rate of about 2%.

Copper IUDs, however, when inserted within five days of unprotected sex, prevent over 99% of pregnancies. The copper IUD for emergency contraception gets recommended by doctors more than the Morning-After Pill, mostly because it can be left in place to provide ongoing birth control. Most women who have it inserted after an “oops” are happy with it later on down the road.

In general, your only two methods of emergency contraception are The Morning After Pill and a copper IUD. Either way, the sooner it’s used, the more likely it’ll work.