This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Contraceptives, Subject name: Gynaecology & Obstetrics
[{'content': 'In Pearl index formula, Pregnancy rate per HWY: \nSelect and explain. \nA. Total accidental pregnancies x 1200 / Number of women observed x Total months of use of contraceptive, B. Total months of exposure to unintended pregnancy X 1200 / Accidental pregnancies, C. Total accidental pregnancies x 100 / Number of women Total months of exposure to unintended pregnancy, D. Total accidental pregnancies / Total months of exposure to unintended pregnancy', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The use-effectiveness of contraceptive method is expressed as Pearl rate & life table technique. Pearl rate or pearl index was introduced by Raymond Pearl in 1932 The rate is expressed as pregnancy rate per 100 women years (HWY) & calculated as Pregnancy rate per HWY = Total accidental pregnancies x 1200/ Number of women observed x Total months of use of contraceptive 1200 is the number of months in 100 years', 'role': 'assistant'}]