The Pearl Index

The Pearl Index (PI) is a measure of the safety of contraceptives. Safe in this context means how well the contraceptive works to prevent pregnancy. The method is named after the scientist Pearl, who first introduced it around 1934.

The Pearl Index is based on the number of unintended pregnancies among one hundred users of the contraceptive over a period of 12 menstrual cycles (approximately one year). The lower the Pearl Index, the better the contraceptive works to prevent unintended pregnancies.

Usually, two indices can be found for each contraceptive, one for perfect use and the other for typical use.
Perfect use refers to use as intended, e.g. taking the pill every day, at the same time as nothing happens that reduces its safety. If pregnancy occurs, this is called a method failure.
Regular use refers to the use of the contraceptive on a daily basis, as some types are easier to use than others. For example, the hormonal IUD is inserted and does not need to be thought about, while it regularly happens that the user forgets to take the pill at the right time. If pregnancy occurs in this case, it is called a user failure.

Please note that the Pearl Index only measures the safety of the contraceptive in regards to pregnancies. It does not take into account whether and to what extent the contraceptive in question protects against sexually transmitted diseases. Remember that the condom is the best protection against sexually transmitted diseases.