Ella prevents about 98% of expected pregnancies when taken within five days of unprotected sex, with an overall pregnancy rate of roughly 1.9% across large clinical trials. That makes it one of the most effective pill-based emergency contraceptives available, and unlike other options, its effectiveness holds remarkably steady across the full five-day window.
How Well Ella Works by the Numbers
In pooled data from Phase III clinical trials involving over 2,000 women, 1.9% became pregnant after taking Ella. That number stayed consistent regardless of when they took it. Women who used Ella between 48 and 72 hours after sex had a pregnancy rate of 2.3%, those between 72 and 96 hours had a rate of 2.1%, and those who waited until 96 to 120 hours still had a rate of just 1.3%. This flat effectiveness curve is one of Ella’s biggest advantages: waiting a few days doesn’t meaningfully reduce how well it works.
The single biggest factor that did increase pregnancy risk wasn’t timing. It was having additional unprotected sex after taking Ella. Women who had further unprotected intercourse in the same cycle were four times more likely to become pregnant. Among women who were not obese and did not have additional unprotected sex, the pregnancy rate dropped to 1.3%.
How Ella Compares to Plan B
Ella outperforms levonorgestrel (the active ingredient in Plan B and its generics) at every time point. It is more effective at preventing ovulation and pregnancy at one, three, and five days after unprotected sex. The gap widens as time passes: Plan B’s effectiveness drops significantly after 72 hours, while Ella maintains its protection through the full 120-hour window.
Ella also has a meaningful advantage for women with higher body weight. Research has found that Plan B becomes no more effective than a placebo at a BMI of 26 or above. Ella maintains some efficacy up to a BMI of 35, though obese women are still about twice as likely to experience a failure compared to non-obese women.
How Ella Prevents Pregnancy
Ella works primarily by blocking or delaying ovulation. It binds to progesterone receptors and shuts down the molecular signals that trigger the follicle to rupture and release an egg. What makes it more powerful than Plan B is its ability to work even after the hormonal surge that normally triggers ovulation has already begun. Plan B cannot stop ovulation once that surge starts, but Ella can, as long as it’s taken within roughly six hours of the surge. After that six-hour window, its ability to block ovulation drops sharply.
In animal studies, Ella didn’t interfere with how the egg develops or matures inside the follicle. Instead, it precisely targeted the rupture step, preventing the follicle from opening. The follicle looked normal in every other way but simply never released the egg. This specificity is what allows Ella to intervene later in the ovulation process than other emergency contraceptive pills can.
If ovulation has already occurred, Ella may also thin the uterine lining, which could make implantation less likely. However, its primary and most well-documented mechanism is preventing ovulation from happening in the first place.
Body Weight and Effectiveness
Weight matters for emergency contraception, and this is an area where Ella has a clear edge. For women with a BMI over 26, levonorgestrel-based pills lose much of their effectiveness. Ella continues to work up to a BMI of about 35, making it the better pill option for women in the overweight and moderately obese range. That said, obese women using Ella still face roughly double the failure rate compared to non-obese women. For women above a BMI of 35, a copper IUD inserted within five days remains the most reliable emergency contraceptive option.
Restarting Birth Control After Taking Ella
This is a detail many people miss: you should wait five full days before starting or restarting hormonal birth control after taking Ella. That means pills, the patch, the ring, injectables, and implants should all be delayed until the sixth day after you take it. Starting hormonal contraception sooner can reduce the effectiveness of both Ella and the birth control itself, because Ella works by blocking progesterone receptors and hormonal contraceptives deliver progestin, which competes with that mechanism.
During the waiting period and for seven days after restarting hormonal contraception (two days for progestin-only pills), you’ll need to use a backup method like condoms. If you want to use a copper IUD instead, there’s no waiting period since it doesn’t involve hormones.
Side Effects
Ella’s side effects are generally mild and temporary. The most common ones include headache, nausea, and abdominal pain. Your next period may come a few days earlier or later than expected, and some women notice changes in flow. These shifts are temporary and resolve on their own within one cycle.
For breastfeeding mothers, guidance has recently changed. As of early 2025, the UK’s Faculty of Sexual and Reproductive Healthcare concluded that there is no need to avoid breastfeeding after taking a single dose of Ella. Previous guidelines had recommended pumping and discarding breast milk for a week, but a review of the evidence found this precaution unnecessary.
When Ella Won’t Work
Ella is not effective if you are already pregnant, and it will not harm an existing pregnancy. It is also not an abortion pill, despite working on the same type of receptor. The dose used for emergency contraception is far lower than what is used in other medications that affect pregnancy.
Ella is also less effective if ovulation happened more than six hours before you take it. Because there’s no practical way to know exactly when you ovulate, taking it as soon as possible still gives you the best odds, even though the overall pregnancy rate stays stable across the five-day window. The stability in those numbers reflects averages across many women at different points in their cycles. For any individual, earlier is still better because it increases the chance of catching ovulation before it happens.