Pearl Index - Calculation and Usage

Calculation and Usage

Three kinds of information are needed to calculate a Pearl Index for a particular study:

• The total number months or cycles of exposure by women in the study.
• The number of pregnancies.
• The reason for leaving the study (pregnancy or other reason).

There are two calculation methods for determining the Pearl Index:

In the first method, the relative number of pregnancies in the study is divided by the number of months of exposure, and then multiplied by 1200.

In the second method, the number of pregnancies in the study is divided by the number of menstrual cycles experienced by women in the study, and then multiplied by 1300. 1300 instead of 1200 is used on the basis that the length of the average menstrual cycle is 28 days, or 13 cycles per year.

The Pearl Index is sometimes used as a statistical estimation of the number of unintended pregnancies in 100 woman-years of exposure (e.g. 100 women over one year of use, or 10 women over 10 years). It is also sometimes used to compare birth control methods, a lower Pearl index representing a lower chance of getting unintentionally pregnant.

Usually two Pearl Indexes are published from studies of birth control methods:

• Actual use Pearl Index, which includes all pregnancies in a study and all months (or cycles) of exposure.
• Perfect use or Method Pearl Index, which includes only pregnancies that resulted from correct and consistent use of the method, and only includes months or cycles in which the method was correctly and consistently used.