This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Family Planning, Subject name: Social & Preventive Medicine
Pearls index? 
Select and explain. 
A. Per 100 woman years, B. Per 10 woman years, C. Per 1000 woman years, D. Per 50 woman years
A: Per 100 woman years
Answer: A: Per 100 woman years 
Explanation: Ans. is 'a' i.e., Per 100 woman years Pearl indexo Contraceptive methods are evaluated by Pearl indexo Pearl index is "failure rate per hundred women years of exposure (HWY)"Total accidental pregnanciesFailure rate per HWY = ------------------------------------------------------------- x 1200Total months of exposureo In applying the above formula, the total accidental pregnancies shown in the numerator, must include every known conception, whatever its outcome. The factor 1200 is the number of months in 100 yrs.o A failure rate of 10 per HWY would mean that in the lifetime of an average women about one fourth or 2.5 accidental pregnancies would result, since the average fertile period of a women is about 25 yrs.