Question 1 · Medium
In a trial of a new contraceptive, 200 women each use the method for 18 months, giving 3,600 woman-months of exposure, and 6 accidental pregnancies occur. What is the Pearl index of the method?
- A3 per 100 woman-years
- B36 per 100 woman-years
- C0.17 per 100 woman-years
- D2 per 100 woman-years
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Answer: D · 2 per 100 woman-years
The Pearl index is the number of accidental pregnancies x 1,200, divided by the total months of exposure, giving failures per 100 woman-years. Here it is 6 x 1,200 / 3,600 = 2 per 100 woman-years. The factor 1,200 is 100 women x 12 months. Its weakness is that failure rates fall the longer a method is used, so life-table analysis gives a fairer comparison of methods.
- A: 3 is pregnancies per 100 women, which ignores the 18 months of use.
- B: 36 comes from dividing by the number of women instead of the months.
- C: 0.17 comes from using 100 instead of 1,200 in the formula.