If sperm has entered your body, pregnancy is not immediate, and you still have a window of time to act. Sperm can survive inside the reproductive tract for 3 to 5 days, waiting for an egg to be released. Emergency contraception works by preventing or delaying that egg release, effectively closing the window before fertilization can happen.
Why You Still Have Time
Pregnancy doesn’t begin the moment sperm enters your body. After sex, sperm travel through the cervix and uterus into the fallopian tubes, where they can remain alive for up to five days. Pregnancy only occurs if a live sperm meets a freshly released egg in that narrow window around ovulation. This biological delay is exactly what emergency contraception exploits: if ovulation can be postponed until the sperm have died, pregnancy is prevented.
Emergency Contraception Pills
Two types of emergency contraception pills are available, and they differ in how long they work and how you get them.
Levonorgestrel pills (sold under brand names like Plan B) are available over the counter without a prescription and with no age restrictions. They work best when taken within 72 hours (three days) of unprotected sex, though sooner is always better. These pills reduce the risk of pregnancy by up to 90% for a single act of unprotected sex. They work by delivering a high dose of a synthetic hormone that delays ovulation.
Ulipristal acetate pills (sold as ella) require a prescription but remain effective for up to 120 hours (five days) after sex. This is the better oral option when more time has passed, and it also performs significantly better for people at higher body weights.
Both types work by interfering with ovulation. Neither one affects an existing pregnancy or prevents a fertilized egg from implanting.
Body Weight Affects Which Pill Works
This is one of the most important and least discussed factors in emergency contraception. Levonorgestrel pills lose effectiveness as body weight increases. People with a BMI over 30 have more than four times the risk of the pill failing compared to those in a normal BMI range. At a body weight of 70 kg (about 154 pounds), levonorgestrel appears to hit a ceiling of effectiveness, and at 80 kg (about 176 pounds) or more, it may provide no meaningful protection at all.
Ulipristal acetate continues to work at higher body weights, making it the better oral choice for people in this range. If your weight is a concern, a copper or hormonal IUD is the most reliable option regardless of body size.
IUDs as Emergency Contraception
An intrauterine device (IUD) is the most effective form of emergency contraception available. The copper IUD has long been the standard: inserted by a clinician within five days of unprotected sex, it creates an environment in the uterus that is hostile to sperm and prevents fertilization. In clinical studies, the pregnancy rate with a copper IUD used for emergency purposes is close to zero.
A hormonal IUD containing levonorgestrel now appears to work just as well. In an NICHD-funded study of 638 women randomly assigned to receive either a hormonal or copper IUD for emergency contraception, just one pregnancy occurred in the hormonal IUD group and none in the copper group. People in the study also reported less discomfort with the hormonal version.
The major advantage of choosing an IUD is that once it’s placed, it doubles as your ongoing birth control for years. The tradeoff is that it requires a clinical appointment, which may be harder to access on short notice.
What to Expect Afterward
Emergency contraception commonly causes short-term changes to your menstrual cycle. About 15% of people experience some spotting or light bleeding within a week of taking the pill, which is not a period and not a sign of pregnancy. Your next actual period may arrive earlier or later than expected. In studies tracking cycle changes after levonorgestrel, roughly 21% of users had their cycle shorten by two or more days, while 24% saw it lengthen by two or more days. If you took the pill before ovulation, your period tends to come a bit early. If you took it after ovulation, expect it a couple of days late.
These disruptions are temporary. Cycle length and flow typically return to normal by the following month. Other common side effects include nausea, headache, and fatigue, which usually resolve within a day or two.
When to Take a Pregnancy Test
If your period arrives within a week of when you expected it, emergency contraception very likely worked. If your period is more than a week late, take a pregnancy test. If three weeks pass from the date you used emergency contraception and you still haven’t gotten your period, a pregnancy test is strongly recommended. You can also test earlier if you start feeling pregnant, as home tests are generally accurate by the time your period is late.
Resuming Regular Birth Control
If you took levonorgestrel (Plan B), you can start or resume hormonal birth control right away. If you took ulipristal acetate (ella), the situation is different: clinical guidelines recommend waiting five days before starting or resuming hormonal contraception, because hormonal birth control can interfere with how ulipristal works. During that five-day gap, and for seven days after restarting your regular method, use condoms or avoid sex entirely. This 12-day stretch of backup protection is important for ensuring both the emergency and ongoing contraception function properly.
If you had an IUD placed as emergency contraception, your ongoing birth control is already handled. The copper IUD provides protection for up to 10 years, and hormonal IUDs last 3 to 8 years depending on the type.