**Mechanism of Emergency Contraceptive Pills**

Emergency contraceptive pills (ECPs), commonly known as morning-after pills, are hormonal medications used to prevent pregnancy after unprotected sexual intercourse or contraceptive failure. They work primarily by preventing or delaying ovulation, fertilization, or implantation of a fertilized egg in the uterus. Let's explore the mechanism of action of ECPs in detail:

**1. Progestin-only pills (POP):**
- Progestin-only emergency contraceptive pills contain a synthetic hormone called progestin, which is similar to the natural hormone progesterone produced by the ovaries.
- When taken within 72 hours after unprotected sex, POPs work primarily by inhibiting or delaying ovulation. By suppressing the release of an egg from the ovary, they prevent fertilization from occurring.
- Progestin also thickens the cervical mucus, making it difficult for sperm to reach the egg.
- Additionally, POPs may alter the lining of the uterus, making it less receptive to implantation.

**2. Combined oral contraceptives (COCs):**
- Combined emergency contraceptive pills contain both progestin and estrogen hormones.
- Like POPs, COCs primarily work by inhibiting or delaying ovulation, preventing the release of an egg from the ovary.
- Estrogen in COCs also helps to prevent pregnancy by suppressing the hormone follicle-stimulating hormone (FSH), which is responsible for stimulating the growth and development of eggs in the ovaries.
- Similar to POPs, COCs thicken the cervical mucus, making it difficult for sperm to reach the egg.
- Additionally, COCs alter the lining of the uterus, reducing the chance of implantation.

**3. Ulipristal acetate (UPA):**
- UPA is a selective progesterone receptor modulator that is available as an emergency contraceptive pill.
- It works by delaying ovulation and inhibiting the release of an egg from the ovary.
- UPA also alters the lining of the uterus, making it less receptive to implantation.

It is important to note that emergency contraceptive pills should be taken as soon as possible after unprotected sex to maximize their effectiveness, ideally within 72 hours but still providing some protection within 120 hours. ECPs are not intended for regular use as a primary contraceptive method and are most effective when used infrequently. It is always recommended to consult a healthcare professional for personalized advice regarding emergency contraception.