Morning-after pills, often called emergency contraception, prevent pregnancy after unprotected sex or contraceptive failure. They are not abortion pills; instead, they work to stop pregnancy from occurring. Their effectiveness, particularly around ovulation, is a common question.
How Morning-After Pills Work
Morning-after pills primarily delay or inhibit ovulation, the release of an egg from the ovary. By preventing egg release, these pills ensure no egg is available for fertilization by sperm.
Two main types are widely available: levonorgestrel and ulipristal acetate. Levonorgestrel works by suppressing the surge of luteinizing hormone (LH), necessary for ovulation. Ulipristal acetate can also delay ovulation even if the LH surge has already begun, offering a slightly longer window of effectiveness than levonorgestrel.
Ovulation and Pregnancy Risk
Ovulation is when a mature egg is released from the ovary, ready to be fertilized. The egg remains viable for fertilization for approximately 12 to 24 hours.
Sperm can survive within the female reproductive tract for up to five days. This means unprotected sex several days before ovulation can still lead to pregnancy. The “fertile window” is the period when conception is most likely, defined by the combination of viable sperm and a viable egg.
Pill Effectiveness Relative to Ovulation
The effectiveness of morning-after pills depends on when they are taken relative to a person’s ovulation cycle. If taken before the luteinizing hormone (LH) surge that signals impending ovulation, both levonorgestrel and ulipristal acetate pills are highly effective by preventing or delaying the LH surge.
Once the LH surge has begun, indicating ovulation is imminent, levonorgestrel’s effectiveness significantly diminishes. Ulipristal acetate, however, retains some ability to delay ovulation even after the LH surge has started, making it a more effective option in this window before egg release.
Morning-after pills are generally not effective once ovulation has occurred and the egg has been released. Their primary mechanism is to prevent egg release; if the egg is already present, they cannot prevent fertilization. These pills do not terminate an existing pregnancy and are not effective if a fertilized egg has already implanted.
Key Factors for Pill Efficacy
The timing of administration significantly influences morning-after pill effectiveness. Taking the pill as soon as possible after unprotected sex greatly increases its chances of preventing pregnancy. Delaying administration reduces its efficacy.
Body weight can also influence effectiveness. Some research suggests levonorgestrel-based pills may be less effective in individuals with a higher body mass index (BMI). Drug interactions are another consideration, as certain medications, such as some anti-seizure drugs or St. John’s wort, can reduce emergency contraception’s effectiveness.
A morning-after pill only addresses the unprotected sexual act that occurred before taking it. Any subsequent acts of unprotected sex after taking the pill are not covered and could still result in pregnancy. Therefore, ongoing contraceptive methods should be used following emergency contraception.