Emergency contraception (EC), commonly known as the morning-after pill, is a method used to prevent pregnancy after unprotected sexual intercourse or contraceptive failure. There are two primary types of hormonal emergency contraceptive pills available: Levonorgestrel, a synthetic progestin, and Ulipristal Acetate, a selective progesterone receptor modulator. Both interrupt the reproductive process before a pregnancy can begin. The effectiveness of these pills is directly tied to the timing of their use within the menstrual cycle. The core question is whether these hormonal methods remain effective once the body has already released an egg (ovulation).
How Emergency Contraception Prevents Pregnancy
The primary way hormonal emergency contraceptive pills function is by interfering with the body’s natural cycle to prevent the release of an egg. The hormones disrupt the carefully timed signals between the brain and the ovaries. Specifically, they suppress the surge of Luteinizing Hormone (LH), the hormonal trigger required for the ovary to release a mature egg.
Levonorgestrel, found in pills like Plan B, works by delaying this LH surge, which postpones or inhibits ovulation. If the egg is not released, there can be no fertilization by sperm, which can survive in the reproductive tract for up to five days. Ulipristal Acetate is even more effective at suppressing or delaying ovulation, especially when taken closer to the expected LH peak.
These pills are not abortifacients; they work to prevent a pregnancy from starting. They do not terminate an established pregnancy, which begins with the implantation of a fertilized egg into the uterine lining. Their mechanism of action focuses on the pre-fertilization stage.
The Critical Timing of Ovulation and Pill Effectiveness
The efficacy of hormonal emergency contraception is strongly dependent on whether the pill is taken before ovulation has occurred. Once the egg is released, the primary mechanism of the pills—preventing that release—becomes moot. Levonorgestrel-based pills are highly ineffective if taken after the LH surge has begun, and they will not prevent pregnancy if ovulation has already passed.
Ulipristal Acetate offers a wider window of effectiveness, remaining active for up to 120 hours (five days) after unprotected intercourse, compared to the 72-hour window for Levonorgestrel. However, even Ulipristal Acetate shows a significant decrease in efficacy once ovulation is confirmed. Studies indicate that Ulipristal Acetate is more successful at preventing pregnancy when administered during the pre-ovulatory phase.
The reason for this sharp drop in effectiveness is biological: hormonal emergency contraceptive pills do not prevent implantation. Once a sperm has fertilized an egg, the pills cannot stop it from attaching to the uterine wall. Therefore, if the egg has been released and fertilization is possible, the hormonal methods lose their ability to prevent a pregnancy.
What to Do When Traditional Pills Are Not Effective
When a person knows or suspects that ovulation has already passed, rendering hormonal EC pills unreliable, the most effective emergency option is the Copper Intrauterine Device (IUD). The Copper IUD is the gold standard for emergency contraception, boasting an efficacy rate of over 99%.
Unlike the morning-after pill, the Copper IUD’s mechanism of action is not dependent on inhibiting ovulation. The device, inserted into the uterus by a healthcare provider, releases copper ions that are toxic to both sperm and eggs, preventing fertilization. This makes it a viable option even if an egg has already been released.
The Copper IUD also creates a sterile inflammatory reaction in the uterine lining, which prevents the implantation of any fertilized egg. This dual action allows it to remain effective even after ovulation. For use as emergency contraception, the device must be inserted within 120 hours (five days) of unprotected sexual intercourse.
Once inserted, the Copper IUD provides highly effective, long-acting contraception for up to ten years. This makes it an ideal choice for individuals who want both immediate emergency coverage and ongoing birth control. Consulting a healthcare provider immediately following unprotected sex is the fastest way to determine if the Copper IUD is the most appropriate choice based on the timing of the menstrual cycle.