Emergency contraception (EC), often referred to as the morning-after pill (MAP), is a medication used to prevent pregnancy shortly after unprotected sex or contraceptive failure. The pills contain hormones that disrupt the biological process of conception. While highly effective when used correctly, no contraceptive method is 100% effective. Several factors can significantly reduce the pill’s ability to prevent pregnancy.
Understanding How Emergency Contraception Works
The primary function of emergency contraceptive pills is to prevent a pregnancy from ever starting. The two main types of oral EC, levonorgestrel (LNG) and ulipristal acetate (UPA), work by delaying or stopping ovulation—the release of an egg from the ovary. If an egg is not released, conception cannot occur.
The pills are not designed to terminate an existing pregnancy. If a fertilized egg has already implanted in the uterine wall, the morning-after pill will not cause an abortion. It acts as a preventative measure that must be taken before conception. The pill’s effectiveness is closely tied to the individual’s menstrual cycle timing at the time of use.
Primary Factors That Reduce Efficacy
The success of the morning-after pill depends on several variables. Understanding these factors is important for maximizing its effectiveness. The most common reasons for failure relate to the timing of use, the user’s body weight, and interactions with other medications. Failing to address these variables can significantly increase the risk of pregnancy.
Timing of Administration
Emergency contraception works by interfering with the ovulatory process, so taking the pill as quickly as possible after unprotected sex is crucial. Levonorgestrel-based pills, such as Plan B, are most effective when taken within the first 72 hours (three days) of unprotected intercourse. Levonorgestrel effectiveness declines sharply with each passing day, though it can be used up to 120 hours.
Ulipristal acetate (UPA), sold under the brand name Ella, is effective for a full 120 hours (five days) after unprotected sex. UPA is generally considered more effective than LNG, particularly when a longer time has passed. This longer window of efficacy is attributed to UPA’s ability to delay ovulation even closer to the point when the egg is about to be released.
Body Weight and BMI
Clinical data suggests that the effectiveness of levonorgestrel-based emergency contraception is reduced in individuals with a higher body weight or Body Mass Index (BMI). Studies show a significant drop in efficacy for levonorgestrel when a person weighs approximately 70 to 75 kilograms. For individuals weighing 80 kilograms or more, the risk of pregnancy becomes comparable to not using any contraception at all.
This reduced effectiveness is thought to be due to how the drug is distributed and metabolized, resulting in lower circulating drug levels. Ulipristal acetate (UPA) maintains a higher level of efficacy in individuals with a higher BMI compared to LNG, though its effectiveness may also decrease slightly. For those with a higher body weight, a healthcare provider may recommend UPA or the copper intrauterine device (IUD), which is unaffected by weight and is the most effective form of EC overall.
Drug Interactions
Certain medications can reduce the efficacy of both levonorgestrel and ulipristal acetate by speeding up the metabolism of the pill’s hormones. These drugs are known as hepatic enzyme inducers, causing the liver to break down the active ingredients faster than usual. If the drug is metabolized too quickly, the hormone level in the bloodstream may not be high enough to prevent ovulation.
Common interacting medications include the antibiotic Rifampin (used to treat tuberculosis) and certain anti-seizure medications like carbamazepine or phenytoin. The herbal supplement St. John’s Wort also acts as an enzyme inducer and can interfere with emergency contraception. Individuals taking these medications should consult a healthcare provider. They may need to consider a different method of emergency contraception, such as the copper IUD, or potentially take a higher dose of the pill.
Next Steps If You Suspect Pregnancy
After taking the morning-after pill, the only way to confirm its success is the arrival of the next menstrual period. The pill commonly disrupts the menstrual cycle, causing the next period to be a few days earlier or up to a week later than expected. This delay warrants attention but should not immediately cause alarm.
If your period is delayed by more than seven days, or if it is much lighter than usual, take an at-home pregnancy test. For individuals with irregular cycles, a pregnancy test should be taken 21 days after the unprotected sexual intercourse. Pregnancy tests detect the presence of the human chorionic gonadotropin (hCG) hormone, which is only produced after implantation.
If the test result is positive, or if you experience symptoms like persistent nausea or unusual abdominal pain, contact a healthcare provider immediately. A medical professional can confirm the pregnancy, check for potential complications like an ectopic pregnancy, and provide counseling regarding available options. The morning-after pill does not pose a risk to a developing fetus if pregnancy occurs despite its use.