## **Core Concept**
The Pearl Index, also known as the Pearl rate, is a commonly used measure to evaluate the effectiveness of contraceptive methods. It is defined as the number of failures (pregnancies) per 100 woman-years of exposure. In simpler terms, it represents the number of pregnancies that occur per 100 women in one year while using a particular contraceptive method.

## **Why the Correct Answer is Right**
To calculate the Pearl Index, we use the formula: Pearl Index = (Number of pregnancies / Number of women) * 100 / Duration of study in years. Given that 5 women became pregnant out of 100 over a period of 20 months, we first convert 20 months into years: 20 months = 20/12 years = 1.6667 years. Then, we calculate the Pearl Index as follows: Pearl Index = (5 / 100) * 100 / 1.6667 = 3.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a calculation that does not match the provided numbers and the correct formula for the Pearl Index.
- **Option B:** Similarly, this option does not align with the correct calculation based on the given data.
- **Option D:** This option also does not correspond to the correct Pearl Index calculation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Pearl Index is a useful but imperfect measure, as it does not account for variables like age, parity, and exposure time variability. For oral contraceptive pills (OCPs), a commonly cited Pearl Index is around 0.1-0.3 for typical use, but this can vary.

## **Correct Answer Line**
**Correct Answer: C. 3.**