## Core Concept
The Pearl Index, also known as the Pearl rate, is a commonly used measure to evaluate the effectiveness of a contraceptive method. It represents the number of failures (accidental pregnancies) per 100 woman-years of exposure or usage.

## Why the Correct Answer is Right
The Pearl Index is calculated as the number of unintended pregnancies divided by the number of woman-years of exposure. To make this rate more understandable and comparable across different studies, it is often expressed per 100 woman-years. This means that if a contraceptive method has a Pearl Index of 1, it implies that there was 1 pregnancy for every 100 women using that method over one year. Therefore, the correct completion of the sentence is "100 woman-years."

## Why Each Wrong Option is Incorrect
- **Option A:** 1000 woman-years - This is incorrect because the standard expression of the Pearl Index is per 100 woman-years, not 1000.
- **Option B:** 1000 women - This option is incorrect because the Pearl Index takes into account the duration of usage (woman-years), not just the number of women.
- **Option C:** 100 women - This option is incorrect for similar reasons as option B; it doesn't account for the duration of usage.
- (No need to discuss the correct answer here)

## Clinical Pearl / High-Yield Fact
A lower Pearl Index indicates a more effective contraceptive method. For example, the Pearl Index for the combined oral contraceptive pill is around 0.1-0.3, indicating that there are 0.1 to 0.3 pregnancies per 100 woman-years of use.

## Correct Answer: B. 100 woman-years.