<prompt> Pearl-index for accidental pregnancies failure per:10 women-years of exposure12 women-years of exposure100 women - years of exposure120 women - years of exposure  <response>: 100 - women - year of exposure: Ref: 439-Park 20thPEARL - INDEX defined as the number of failure per 100 women years of exposure HWYFailure rate per HWY Total accidental pregnanciesTotal months of exposurex l200 The total accidental pregnancies shows in the Numerator must include every known conception. The factor 1200 is the number of months in 100 years The total month of exposure in the Denominator is obtaining by deducing from the period under review of 10 months for a full term pregnancy and 4 months for an abortion The pearl index is usually based on a specific exposure usually one year 