Pearl Index

Determines how effective a contraception method is based on pregnancy occurrence during study period.

In the text below the calculator there is more information on the usage of the Pearl Index.

The Pearl Index evaluates the effectiveness of a contraceptive method based on the number of pregnancies that occur during a studied period. This becomes a statistical estimation of pregnancy risk per year.

The effectiveness or the failure rate of a contraceptive method should be the main factor taken into account when choosing contraception.

The Pearl index is based on the following formula devised by Raymond Pearl in 1933:

Pearl index = (No. of pregnancies x 12) x 100 / (No. of women in the study x Duration of study in months)

For example, if 100 use the studied contraceptive method for 12 months and 3 unintended pregnancies occur, the Pearl index is 3.

The lower the Pearl Index, the higher the effectiveness of the tested contraceptive method.

Number of women in the study
Number of pregnancies
Number of months
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Variables and formula

The Pearl index is particularly useful in clinical research trials which check the effectiveness of current or new birth control methods.

This is a method created by Raymond Pearl in 1933 and consists of three variables and a formula:

■ Number of women in the study;

■ Number of unintended pregnancies;

■ Number of months.

The former refers to either the duration of the study in months or cycles of exposure by women in the study.

The Pearl formula is:

Pearl index = (No. of pregnancies x 12) x 100 / (No. of women in the study x Duration of study in months)

If during a 16-month study on 235 women, 5 unintended pregnancies occur, the contraceptive method tested has a Pearl index of approximately 1.6 (1.59574468085).

The higher the Pearl index, the lower the effectiveness/ the higher the failure rate of the contraceptive method.


Pearl index guidelines

The Pearl index offers information about the effectiveness of specific birth control methods.

There are other factors that affect this statistical estimation of pregnancy risk, such as personal usage and how thoroughly manufacturer instructions are followed.

The index is often used when comparing different contraceptive methods:

Contraceptive method Pearl index
Condom 3-12
Birth Control Pill 0.1-1
Intrauterine Device IUD 0.1-1.5
Cervical cap 4-20
Sympto-Thermal Method 0.5-2
Standard Days Method 4-5
Sterilization 0.1-0.4

The main limitation of the method is the fact that it relies on the observation of only a sample population, therefore, the results may vary if extending the research with different populations. Also, risk of sexually transmitted diseases is not accounted for.

It is important to assume a constant failure rate independent on personal conditions (such as female age and degree of fertility), which is something beyond the purpose of the Pearl index.

On a similar note, the duration of the study may also impact the result as it was statistically found that the longer a couple uses a certain method, the lower the registered failure rate.

Risk of sexually transmitted diseases is not accounted for in any of the calculations.


Original source

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

Other references

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

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

Specialty: Obstetrics Gynecology

System: Reproductive

Objective: Evaluation

Type: Index

No. Of Variables: 3

Year Of Study: 1933

Article By: Denise Nedea

Published On: May 25, 2017 · 01:05 PM

Last Checked: May 25, 2017

Next Review: May 25, 2023