How emergency contraception works
The three available emergency contraception (EC) options work mainly by delaying or blocking ovulation, or in the case of the copper IUD, by preventing fertilization and implantation.
- Levonorgestrel (Plan B and generics): over the counter, most effective in the first 24 hours, approved up to 72 hours.
- Ulipristal acetate (Ella): prescription in most places, effective up to 120 hours, the strongest oral option closer to ovulation.
- Copper IUD (ParaGard): inserted at a clinic within 5 days, >99% effective, and ongoing contraception for years afterward.
What to do next
Track your next period. If it's more than a week late, take a test - the pregnancy test calculator can help you time it. EC doesn't protect against future cycles; if you need ongoing contraception, the birth control finder can help you compare options.
Frequently asked questions
How quickly should I take Plan B?
The sooner the better. Levonorgestrel (Plan B and generics) is most effective in the first 24 hours and approved for use within 72 hours. After 72 hours, ulipristal acetate (Ella) or a copper IUD insertion are better options.
Which is most effective?
The copper IUD is by far the most effective emergency contraception - over 99% - when inserted within 5 days. Ulipristal is the most effective oral option. Levonorgestrel is the most widely available but the least effective of the three.
Will it work if I'm already ovulating?
Effectiveness drops sharply once ovulation has already happened. Levonorgestrel mostly works by delaying ovulation; if ovulation already occurred, it's much less effective. Ulipristal can still work right around ovulation, and the copper IUD prevents implantation regardless of where you are in your cycle.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.