The Pearl Index is often used to compare the effectiveness of various methods of contraception.[41] It is expressed as the "number of unintended pregnancies in 100 normally fertile women over the period of one year". Each method of birth control has two Pearl index numbers: * method effectiveness: is the Pearl index number for use under perfect conditions. The method effectiveness Pearl index for the Pill has been measured as low as 0.3 and as high as 1.25, which means that under ideal conditions, anywhere from 0.3 to 1.25 out of 100 users will become pregnant during one year of perfect use (Pearl index = 0.3 to 1.25). * user effectiveness or typical effectiveness: is the Pearl index number for use that is not consistent or always correct. The user effectiveness measured by the Pearl index for the Pill has been measured as low as 2.15 and as high as 8.0, which means that anywhere from 2.15 to 8.0 out of 100 women will become pregnant during the first year of typical use (Pearl index = 2.15 to 8.0).[42][43] If a woman just starting the pill begins taking them within five days of the beginning of her menstrual cycle (the menstrual cycle begins on the first day of red bleeding), she will have pregnancy protection from the very first pill. If a woman begins taking the pill at another time in her menstrual cycle, she must use a different form of contraception for seven days.[44] Many women occasionally forget to take the Pill daily, impairing its effectiveness. Correct use of the pill usually implies taking it every day at the same hour for 21 days, followed by a pause of seven days. Use of other medications can prevent the Pill from working, due to interactions with the metabolism of the hormonal constituents. Diarrhea can also stop the Pill from working, because it causes the hormones to not be properly absorbed by the bowels. http://en.wikipedia.org/wiki/Pearl_Index