Emergency Contraception (EC), often referred to as the “morning after pill,” is a time-sensitive medication used to prevent pregnancy after unprotected sexual intercourse. Its effectiveness hinges entirely on the timing of the menstrual cycle, specifically whether ovulation has already taken place. EC is not an abortion pill; it works exclusively by preventing pregnancy from starting and will not terminate an existing, implanted pregnancy. Immediate timing is paramount, as efficacy drops significantly closer to egg release.
How Emergency Contraception Prevents Pregnancy
The primary mechanism of action for hormonal emergency contraception is to interfere with the body’s ovulatory process. These pills suppress the release of Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus, which prevents the pituitary gland from triggering the Luteinizing Hormone (LH) surge.
The LH surge is the hormonal signal that causes a mature egg to be released from the ovary. By blocking this signal, the egg is never released, and there is no ovum available for sperm to fertilize. Since sperm can survive in the reproductive tract for up to five days, delaying ovulation allows time for the sperm to die off.
Hormonal emergency contraception does not affect a pregnancy once a fertilized egg has implanted in the uterine wall. The drug acts only to prevent fertilization and does not cause changes to the uterine lining that would prevent implantation or increase the rate of miscarriage.
The Role of Ovulation Timing
The effectiveness of hormonal emergency contraception is entirely dependent on where the person is in the ovulatory cycle when the pill is taken. The pill’s function is to delay the release of the egg, meaning it must be taken before the body’s natural processes have gone too far. If the egg has already been released from the ovary—meaning ovulation has occurred—the pill’s primary mechanism of action can no longer be successful.
The window of opportunity is directly related to the Luteinizing Hormone surge. For the pill to work, it must be administered before the LH levels in the blood have reached their peak, which is the point of no return for ovulation. Once the LH surge has peaked, the follicle is committed to rupturing and releasing the egg, and the hormonal medication is rendered ineffective.
Taking the emergency contraceptive pill when ovulation has already happened means the egg is now traveling down the fallopian tube, making it available for fertilization. At this point, the medication cannot reverse the release of the egg or prevent fertilization. Therefore, if a person is certain they are ovulating, the effectiveness of the pill is severely compromised.
Comparing Different Emergency Contraception Options
The two main types of hormonal emergency contraception, Levonorgestrel and Ulipristal Acetate, differ in their ability to interfere with the LH surge. Levonorgestrel-based pills, often sold over the counter, are progestin-only and must be taken before the LH surge begins to be fully effective. Once the surge has started, Levonorgestrel is generally unable to halt the process of ovulation.
Ulipristal Acetate, which requires a prescription, is a selective progesterone receptor modulator that maintains efficacy closer to the time of ovulation. This medication is able to delay follicular rupture even after the LH surge has started, but before it reaches its peak. Due to this unique mechanism, Ulipristal Acetate is considered to be more effective than Levonorgestrel when taken in the days immediately leading up to ovulation.
The most effective form of emergency contraception available is the Copper Intrauterine Device (IUD). The Copper IUD is highly effective because its mechanism of action is distinctly different from the hormonal pills. It works by creating a local inflammatory reaction in the uterus that is toxic to both sperm and eggs, preventing fertilization and implantation. It is the only form of emergency contraception that works reliably even after ovulation has occurred.
Next Steps If Hormonal Emergency Contraception Is Not an Option
If a person suspects or knows that ovulation has already occurred, or if more than five days have passed since unprotected intercourse, hormonal EC is no longer a viable option. In this scenario, the immediate next step is to seek the insertion of a Copper IUD. The Copper IUD can be inserted by a healthcare professional up to five days after the earliest estimated day of ovulation or up to five days after unprotected sex.
This device is nearly 100% effective in preventing pregnancy when used for emergency contraception and can then be left in place for long-term birth control. If the Copper IUD is not chosen, the person should consult a healthcare provider for follow-up and counseling. This consultation will involve planning for a pregnancy test and discussing subsequent options if a pregnancy is confirmed.
These subsequent options could include medical termination, which uses a different drug regimen than emergency contraceptive pills and is designed to end an established pregnancy. A healthcare provider can provide guidance and support throughout this time-sensitive decision process.