Emergency contraceptives (ECs) prevent pregnancy after unprotected sexual intercourse or contraceptive failure. These methods provide an important option for individuals seeking to avoid unintended pregnancy. Understanding how these contraceptives function, particularly in relation to the menstrual cycle, is important for effective use.
Understanding the Ovulatory Process
Ovulation is an event in the menstrual cycle where a mature egg is released from an ovary. This process occurs around day 14 of a 28-day cycle, though timing can vary. Once released, the egg travels down the fallopian tube, where it can be fertilized by sperm. Pregnancy cannot occur without this release.
An egg remains viable for 12 to 24 hours after release, but sperm can survive in the female reproductive tract for up to five days. The “fertile window” includes the day of ovulation and the five days leading up to it. This means sexual intercourse occurring several days before ovulation can still result in pregnancy.
How Emergency Contraceptives Work
Emergency contraceptives work by interfering with the reproductive process to prevent pregnancy. Hormonal emergency contraceptive pills primarily prevent or delay ovulation. This stops an egg from being released, preventing it from meeting sperm. If ovulation is delayed, sperm may die before an egg is released, thus preventing conception.
Some types of emergency contraceptives also influence other physiological processes. For instance, some hormonal ECs can affect cervical mucus, making it thicker and more difficult for sperm to travel. Current evidence indicates that hormonal ECs primarily work before fertilization or implantation occurs and do not interfere with an established pregnancy. The copper intrauterine device (IUD) prevents fertilization and, if fertilization has occurred, prevents implantation.
Emergency Contraceptive Effectiveness Relative to Ovulation
The effectiveness of hormonal emergency contraceptives, such as those containing levonorgestrel (LNG) or ulipristal acetate (UPA), is influenced by the timing relative to ovulation. These pills are most effective when taken before ovulation has occurred, as their primary mechanism is to delay or inhibit the release of an egg. If ovulation has already happened, hormonal EC pills are ineffective.
Levonorgestrel-based ECs are effective in preventing or delaying ovulation when taken before the luteinizing hormone (LH) surge begins, which triggers ovulation. Once the LH surge has started, levonorgestrel is much less effective at preventing ovulation. Ulipristal acetate, however, can delay ovulation even if taken shortly before or during the initial phase of the LH surge, making it effective later in the cycle than levonorgestrel. UPA’s efficacy is higher when administered before ovulation compared to after.
Once ovulation has occurred and an egg has been released, levonorgestrel and ulipristal acetate pills cannot prevent pregnancy. Their main action is to prevent the release of an egg, so if the egg is already available, their purpose is bypassed. The effectiveness of these pills decreases the closer a person is to ovulation, becoming ineffective if taken during or after the egg has been released.
Choosing the Right Emergency Contraceptive Option
Emergency contraceptive options are available, and their effectiveness can vary depending on when they are used in the menstrual cycle. Levonorgestrel pills (e.g., Plan B One-Step) are commonly available over-the-counter and are most effective when taken within 72 hours (three days) of unprotected sex. Their primary action is to delay ovulation, and they are not effective if ovulation has already occurred.
Ulipristal acetate (UPA), known as Ella, is a prescription oral emergency contraceptive. UPA can be taken up to 120 hours (five days) after unprotected sex and is considered more effective than levonorgestrel, particularly if taken closer to ovulation. It works by delaying or inhibiting ovulation, even if the LH surge has begun but not yet peaked.
The copper intrauterine device (IUD) is the most effective form of emergency contraception and differs from hormonal pills. It can be inserted up to five days after unprotected intercourse or the earliest estimated day of ovulation. The copper IUD works by creating a toxic environment for sperm and eggs, preventing fertilization, and can prevent implantation if fertilization has occurred. Unlike hormonal pills, it remains highly effective even if ovulation has taken place.