Understanding Emergency Contraception
Emergency contraceptive pills (ECPs), commonly known as "morning-after pills," are a safe and effective way to prevent pregnancy after unprotected intercourse or contraceptive failure [1.6.2]. It is crucial to understand that they are a form of contraception, not an abortion pill [1.7.4]. Their primary function is to prevent a pregnancy from starting. The World Health Organization (WHO) and other health bodies confirm that ECPs do not interrupt an established pregnancy or harm a developing embryo [1.2.3, 1.7.2]. The effectiveness of these pills is highly dependent on how soon they are taken after the event [1.2.4].
The Primary Mechanism of Action: Delaying Ovulation
The main way emergency pills work is by preventing or delaying ovulation, which is the release of an egg from the ovary [1.3.2]. The pills contain hormones, either a progestin called levonorgestrel or a progesterone receptor modulator called ulipristal acetate, that interfere with the hormonal signals that trigger ovulation [1.2.4, 1.3.1]. If there is no egg for sperm to meet, fertilization cannot occur, and therefore pregnancy is prevented [1.2.4].
Some evidence suggests secondary mechanisms that may contribute to their effectiveness:
- Thickening Cervical Mucus: The hormones can make the cervical mucus thicker, which makes it more difficult for sperm to travel through the cervix and reach an egg [1.2.4, 1.3.6].
- Altering Sperm Function: There's a possibility that the hormones may affect the ability of sperm to bind to an egg [1.3.6].
It is a common misconception that ECPs work by preventing a fertilized egg from implanting in the uterine wall. However, major health organizations state that the primary mechanism is the inhibition of ovulation and that ECPs are not effective once implantation has occurred [1.3.2, 1.3.6].
Types of Emergency Contraceptive Pills
There are two main types of emergency contraceptive pills available, each with different active ingredients and windows of effectiveness [1.2.2].
Levonorgestrel (e.g., Plan B One-Step, My Way)
Levonorgestrel is a synthetic hormone (progestin) that has been used in birth control pills for decades [1.2.4]. It is available over-the-counter without a prescription [1.2.2].
- Time Window: It is most effective when taken as soon as possible and should be used within 72 hours (3 days) of unprotected intercourse [1.4.2]. Its effectiveness decreases over time [1.2.4].
- Effectiveness: Studies show it can reduce the risk of pregnancy by 81% to 90% when taken within the 72-hour window [1.6.3].
- BMI Considerations: Some studies suggest levonorgestrel may be less effective in individuals with a Body Mass Index (BMI) of 25 or higher, and may not be effective for those with a BMI over 30 [1.9.1, 1.9.2].
Ulipristal Acetate (e.g., ella)
Ulipristal acetate is a selective progesterone receptor modulator, which means it can block the effects of progesterone [1.3.1]. It is available by prescription only [1.4.3].
- Time Window: It can be taken up to 120 hours (5 days) after unprotected sex, and its effectiveness remains more consistent throughout this window compared to levonorgestrel [1.4.2, 1.4.4].
- Effectiveness: It is considered the most effective type of ECP, with studies showing pregnancy rates of 1-2% [1.6.4]. It is also more effective than levonorgestrel when taken closer to ovulation [1.3.1].
- BMI Considerations: While its efficacy may also be reduced in individuals with a very high BMI (35 or more), it is generally considered more effective than levonorgestrel for people with a higher BMI [1.9.1, 1.9.2].
Comparison of Emergency Pill Types
| Feature | Levonorgestrel (e.g., Plan B) | Ulipristal Acetate (e.g., ella) |
|---|---|---|
| Active Ingredient | Progestin (Levonorgestrel) [1.2.4] | Selective Progesterone Receptor Modulator (Ulipristal Acetate) [1.3.1] |
| Primary Action | Delays or inhibits ovulation [1.3.1] | Delays or inhibits ovulation, even close to the LH surge [1.3.1] |
| Time Window | Up to 72 hours (3 days) [1.4.2] | Up to 120 hours (5 days) [1.4.2] |
| Prescription | No, available over-the-counter [1.2.2] | Yes, prescription required [1.4.3] |
| Effectiveness | 81-90% effective, decreases with time [1.6.3] | More effective than levonorgestrel, especially between 72-120 hours [1.4.4] |
| BMI Impact | May be less effective with BMI > 25 [1.9.1] | More effective than levonorgestrel in higher BMI, but may be reduced with BMI > 35 [1.9.2] |
Potential Side Effects and Body Changes
After taking an emergency pill, the body experiences a significant, but temporary, hormonal shift. This can lead to several short-term side effects, which typically resolve within a few days [1.2.1, 1.8.2]. There are no known long-term health risks or impacts on future fertility associated with using ECPs [1.8.1, 1.8.2].
Common short-term side effects include:
- Changes to your Menstrual Cycle: Your next period may come earlier or later than expected, and the flow could be heavier or lighter than usual [1.5.2, 1.5.4]. Spotting or light, irregular bleeding between periods is also common [1.5.1].
- Nausea and Vomiting: Some people may feel nauseous [1.5.1]. If you vomit within two hours of taking the pill, you should contact a healthcare provider, as you may need to take another dose [1.2.3].
- Other common effects: Headaches, fatigue, dizziness, breast tenderness, and lower abdominal pain or cramping are also possible side effects [1.2.1, 1.5.2].
Conclusion
Emergency contraceptive pills work by delivering a dose of hormones that primarily acts to prevent or delay ovulation, thus preventing a pregnancy from beginning [1.2.1]. They are a safe backup method and are distinct from abortion pills [1.7.4]. The two main types, levonorgestrel and ulipristal acetate, differ in their effective time windows and how they are accessed [1.4.3]. While they can cause temporary side effects like changes in your menstrual cycle and nausea, there are no known long-term risks to health or fertility [1.8.1]. Understanding how they work, the types available, and their effectiveness can empower individuals to make informed decisions about their reproductive health.
For more detailed information, you can consult an authoritative source like the World Health Organization (WHO).
