Pearl Index Calculator

How does this Pearl index calculator work?

This is a health tool that employs the Pearl rate formula in order to help research in clinical trials in regard to the effectiveness of the tested birth control methods.

This is based on the method of calculation instituted by Raymond Pearl in 1933.

There are three variables taken into account in the Pearl Index Calculator as follows:

■ Number of women in the study;

■ Number of pregnancies – self explanatory;

■ Number of months – meaning the time duration of the study in months or cycles of exposure by women in the study.

The formula used is:

Pearl index = (number of pregnancies x 12) x 100 / (number of women in the study x duration of study in months)

For example, if during a test 100 women use a certain contraceptive method and during a period of 12 months, 2 unintended pregnancies occur, the Pearl index is 2.

It has been statistically established that effectiveness/ failure rate is the main factor taken into account when choosing a particular contraceptive method.

Pearl Index guidelines

The Pearl Index is recorded as a statistical estimation of pregnancy risk per year. There are however other personal usage factors and respecting the manufacturer instructions which can alter the effectiveness of specific birth control methods.

One of the criticisms received by the formula lies on the fact that the calculation is based on the observation of only a sample population, meaning that by extending the test within different populations, even the same contraceptive can perform differently.

At the same time, every study assumes a constant failure rate independent on personal conditions, such as the fact that the most fertile couples will get pregnant first as well as young women before older women.

Studies have also revealed that the longer a couple uses a certain method, the more effective that appears for their particular case, meaning that in general, the longer the study, the lower the registered failure rate. Risk of sexually transmitted diseases is not accounted for in any of the calculations.

The Pearl Index is mainly used in the comparison of contraception methods related to the number of unintentional pregnancies to 100 women years. Some examples include condoms with 2-15, standard days 4-5, IUD intrauterine device 0.2-2, birth control pill 0.1-1 or sterilization with 0.1-0.4.

Beside the Pearl rate, most common failure factors are related to inappropriate or inconsistent usage. This is why the simultaneous use of two contraceptive methods lowers the failure risk consistently.

For example the annual probability of unpredicted pregnancy by using both condoms and spermicides is 0.03%.

References

1) Pearl, Raymond (1933). "Factors in human fertility and their statistical evaluation". Lancet 222 (5741): 607–611

2) Trussell J, Hatcher RA, Cates W Jr, Stewart FH, Kost K. (1990) A guide to interpreting contraceptive efficacy studies. Obstet Gynecol; 76(3 Pt 2):558-67.

3) Trussell J, Portman D. (2013) The Creeping Pearl: Why Has the Rate of Contraceptive Failure Increased in Clinical Trials of Combined Hormonal Contraceptive Pills? Contraception; 88(5): 604–610.

4) Trussell J. (1991) Methodological pitfalls in the analysis of contraceptive failure. Stat Med; 10(2):201-20.

5) Sheps MC. (1966) Characteristics of a ratio used to estimate failure rates: occurrences per person year of exposure. Biometrics; 22(2):310-21.