Pearl's index indicates :

Pearl Index The incidence of a disease or event is often quoted in rate percent per year, for example, the failure rate of contraceptives is quoted using the Pearl Index. The following information is required to calculate the Pearl Index: the number of pregnancies and the total number of months or cycles of exposure of women. The index can then be calculated as follows: number of pregnancies in the study divided by the number of months of exposure and then multiplied by 1200 number of pregnancies in the study divided by the number of menstrual cycles experienced by the women and then multiplied by 1300; 1300 is used instead of 1200 because the average menstrual cycle is 28 days giving 13cycles per year. Ref: Dewhurst's Textbook of Obstetrics & Gynaecology; 8th edition; Chapter 32; Page no: 398