100 women, followed up for 20 months, with OCPs, 5 became pregnant. Calculate the Pearl Index

**Core Concept**
The Pearl Index is a measure of contraceptive efficacy, specifically the number of failures (pregnancies) per 100 women-years of exposure. It's a widely used metric to evaluate the effectiveness of hormonal contraceptives like Oral Contraceptive Pills (OCPs).

**Why the Correct Answer is Right**
To calculate the Pearl Index, we use the formula: (Number of pregnancies × 1200) / Number of women-years of exposure. In this case, 5 pregnancies occurred over 100 women × 20 months (or 1.67 years, approximately). Therefore, the Pearl Index is (5 × 1200) / (100 × 1.67) = 36. This means that for every 100 women using OCPs for 1 year, approximately 36 would become pregnant.

**Why Each Wrong Option is Incorrect**
**Option A:** The calculation involves more than just the number of pregnancies, so the provided answer is incomplete.
**Option B:** This is the correct answer, so it's not incorrect, but rather the correct choice.
**Option C:** The formula for the Pearl Index involves multiplying the number of pregnancies by 1200, not 100.
**Option D:** This option incorrectly assumes the number of pregnancies is the final answer, without taking into account the number of women-years of exposure.

**Clinical Pearl / High-Yield Fact**
Remember the Pearl Index formula: (Number of pregnancies × 1200) / Number of women-years of exposure. This will help you calculate the effectiveness of various contraceptive methods.

**Correct Answer:** B. 36