Emergency contraception reduces the risk of pregnancy primarily by delaying or preventing ovulation, the release of an egg from the ovary. Without an available egg, sperm cannot fertilize anything, and pregnancy doesn’t occur. The specific mechanism depends on which type of emergency contraception is used, and timing plays a major role in how well each option works.
How the Pill Forms Work
There are two types of emergency contraceptive pills available, and they work in slightly different ways. Both target ovulation, but one has a wider window of effectiveness than the other.
The most widely available option contains levonorgestrel, a synthetic form of progesterone. It works by suppressing or delaying the hormonal surge that triggers the ovary to release an egg. If taken early enough in the cycle, before that surge begins, it can effectively postpone ovulation long enough for any sperm in the reproductive tract to die off (sperm typically survive three to five days). However, levonorgestrel becomes much less reliable as ovulation approaches. Studies have shown that when given in the one to two days before expected ovulation, follicular rupture (the physical release of the egg) still occurred in 80 to 92 percent of cycles. In other words, if ovulation is already imminent, this pill often can’t stop it.
The second pill option contains ulipristal acetate, which works differently at the molecular level. Rather than simply adding progesterone-like hormones, it blocks progesterone receptors in the ovary. This gives it the ability to delay egg release even after the hormonal surge has started. In a controlled study, ulipristal prevented follicular rupture for at least five days in 100 percent of cycles when given before the surge began, and in about 79 percent of cycles when given after the surge started but before it peaked. Only after the surge had fully peaked did its effectiveness drop sharply, to about 8 percent. This extended window is why ulipristal acetate reduces pregnancy risk 42 percent more than levonorgestrel when taken within 72 hours, and 65 percent more when taken within the first 24 hours.
How IUDs Work as Emergency Contraception
A copper IUD inserted within five days of unprotected sex is the most effective form of emergency contraception available. Its primary mechanism is different from the pills: copper ions released into the uterus are toxic to sperm. They impair sperm motility and function so severely that fertilization is essentially prevented. In a randomized trial comparing IUD options, zero pregnancies occurred among 328 participants who received a copper IUD, representing 100 percent prevention of expected pregnancies.
More recently, hormonal IUDs containing 52 mg of levonorgestrel have also been studied for emergency use. In the same trial, only one pregnancy occurred among 327 participants who received a hormonal IUD, preventing an estimated 92 to 96 percent of expected pregnancies. While not quite matching the copper IUD’s perfect record, hormonal IUDs offer the advantage of lighter periods and can serve as long-term contraception afterward. Both IUD types have a key practical benefit over pills: they work regardless of where you are in your cycle, because they don’t rely on blocking ovulation.
Why Timing Changes Everything
Emergency contraception prevents roughly 85 percent of pregnancies that would otherwise occur. But that number is an average, and your actual risk reduction depends heavily on two timing factors: when in your cycle the unprotected sex happened, and how quickly you take action afterward.
On a random day in the cycle, the chance of pregnancy from a single act of unprotected sex is about 8 percent. Near ovulation, that probability jumps to as high as 40 percent. Emergency contraceptive pills are most effective when taken well before ovulation, because they need time to suppress the hormonal cascade. If ovulation has already occurred, the pills have little to no effect, since the egg is already available for fertilization. This is why the common advice to take emergency contraception “as soon as possible” isn’t just about convenience. Every hour that passes brings you closer to the point where the pill can no longer intervene.
IUDs don’t face this same limitation as starkly. Because copper acts directly on sperm rather than on ovulation, a copper IUD can still be effective even if inserted after the egg has been released.
Body Weight and Effectiveness
Body weight can reduce how well oral emergency contraception works. Levonorgestrel may be less effective in women over 165 pounds and could lose meaningful efficacy above 176 pounds. The mechanism is straightforward: a larger body volume dilutes the drug concentration in the bloodstream, potentially making the dose insufficient to suppress ovulation.
Ulipristal acetate holds up better at higher weights, though it isn’t immune to the effect. For women with a BMI over 30, the risk of unintended pregnancy with ulipristal is about 2.5 percent, compared to roughly 6 percent with levonorgestrel. That said, medical organizations including the Society of Obstetricians and Gynaecologists of Canada have emphasized that women at higher weights should not be discouraged from using levonorgestrel if it’s their only available option, since some protection is better than none. For those who want the most reliable option regardless of weight, an IUD sidesteps the issue entirely.
What to Expect Afterward
Side effects from emergency contraceptive pills are generally mild and short-lived, typically lasting only a few days. Nausea is the most common complaint. Your next period may arrive up to a week later than expected, which is a direct result of the ovulation delay the medication caused. If your period is more than a week late, a pregnancy test is a reasonable next step.
An IUD insertion involves a brief office visit and can cause cramping during and after the procedure. But it transitions seamlessly into long-term contraception, effective for years, which is one reason many providers consider it the best emergency option for people who also want ongoing protection.