Emergency contraception, often called the morning after pill, prevents pregnancy after unprotected sexual intercourse or when a primary birth control method has failed. These medications are intended for emergency situations and do not function as a regular form of birth control.
How Emergency Contraception Works
Emergency contraceptive pills primarily work by affecting ovulation, the release of an egg from the ovary. The most common types contain either levonorgestrel or ulipristal acetate.
Levonorgestrel-based pills, such as Plan B One-Step, function by delaying or preventing egg release. If an egg is not released, there is no egg available for sperm to fertilize.
Ulipristal acetate pills, known as Ella, also primarily work by delaying or inhibiting ovulation. They can even delay ovulation if taken during the luteinizing hormone (LH) surge, which is the hormonal signal that precedes egg release.
The Process of Ovulation
Ovulation is a phase within the female menstrual cycle where a mature egg is released from an ovary. This event typically occurs around the midpoint of the menstrual cycle, though timing can vary significantly. For an average 28-day cycle, ovulation usually takes place approximately 14 days before the next menstrual period.
Before ovulation, a surge in luteinizing hormone (LH) signals the ovary to release the egg. Once released, the egg travels into the fallopian tube, where it can be fertilized by sperm. An egg remains viable for fertilization for about 12 to 24 hours. Sperm can survive within the female reproductive tract for up to three to five days, meaning the “fertile window” for conception spans several days leading up to and including ovulation.
Emergency Contraception and Ovulation Timing
The effectiveness of emergency contraceptive pills is closely linked to the timing of their administration relative to ovulation. Levonorgestrel-based pills work by delaying or preventing egg release. If ovulation has already occurred before these pills are taken, their primary mechanism of action cannot prevent pregnancy, making them generally ineffective.
Ulipristal acetate (Ella) offers a slightly broader window of effectiveness. It can still delay ovulation even when taken during the initial phase of the LH surge. However, like levonorgestrel, ulipristal acetate is less effective if ovulation has already taken place. Neither type of emergency contraceptive pill will work if fertilization and implantation have already occurred, as they are not designed to interrupt an established pregnancy.
Other Effectiveness Factors and Post-Pill Guidance
Beyond the timing of ovulation, other factors influence the effectiveness of emergency contraceptive pills. Taking the pill as soon as possible after unprotected sexual intercourse significantly increases its chances of success. While levonorgestrel is most effective within 72 hours, ulipristal acetate can be taken up to 120 hours (five days) after unprotected sex; earlier administration is always better.
Body weight may also play a role in the effectiveness of hormonal emergency contraception. Some studies suggest levonorgestrel-based pills may be less effective in individuals who weigh more than 165 pounds or have a body mass index (BMI) over 25. Ulipristal acetate may show reduced effectiveness in individuals weighing over 195 pounds or with a BMI over 30, though data is less conclusive. Individuals should still take emergency contraception if needed and consult a healthcare provider for personalized advice.
If emergency contraceptive pills may not be effective, particularly if ovulation has already occurred or too much time has passed, other options exist. The copper intrauterine device (IUD) is a highly effective form of emergency contraception. It can be inserted up to five days after unprotected sex. Its mechanism of action, which involves inhibiting fertilization and creating an environment hostile to sperm and eggs, means it remains effective even if ovulation has occurred or fertilization has taken place.
Following any emergency contraception use, it is advisable to follow up with a healthcare provider, especially if a period is delayed, to confirm pregnancy status and discuss ongoing birth control methods.