Mechanism of action of oral contraceptive containing a combination of oestrogen-progestin is
## **Core Concept**
The mechanism of action of oral contraceptives containing a combination of estrogen and progestin involves multiple pathways that prevent ovulation, fertilization, and implantation of the embryo. These contraceptives are widely used for birth control and contain synthetic forms of estrogen and progestin.
## **Why the Correct Answer is Right**
The correct answer, **. Prevention of ovulation**, is right because the primary mechanism by which combination oral contraceptives (COCs) work is by inhibiting the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. This leads to a decrease in the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland. With reduced LH and FSH levels, **ovulation is prevented** because the surge in LH that typically triggers ovulation does not occur. Additionally, COCs cause changes in the cervical mucus, making it thicker and more impenetrable to sperm, and alter the endometrium, making it less receptive for implantation.
## **Why Each Wrong Option is Incorrect**
- **Option A: . Thickening of cervical mucus** is a mechanism of action but not the primary one. While it does occur and contributes to the contraceptive effect by impeding sperm penetration, it is not the main mechanism.
- **Option B: . Implantation of blastocyst** is actually what COCs prevent indirectly. They do not directly cause implantation but make the endometrium unfavorable for it.
- **Option C: . Enhancement of ovulation** is incorrect because COCs actually prevent ovulation rather than enhance it.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that combination oral contraceptives have a **high efficacy rate** when used correctly, with a failure rate of less than 1% in the first year of use. However, their effectiveness can be reduced by factors such as missed pills, drug interactions (e.g., with certain antiepileptics or rifampicin), and certain medical conditions.
## **Correct Answer: D. Prevention of ovulation**