Contraceptive Efficacy Estimate
Enter study data or clinical experience to compute the Pearl Index — pregnancies per 100 woman-years — for a contraceptive method.
Exposure time
225.00 woman-years
Pearl Index
0.89
Pregnancies per 100 woman-months
0.07
How to Use This Calculator
Collect contraceptive cohort data
Record the number of participants, total observation period, and unintended pregnancies.
Enter total exposure months
Multiply participant count by months of participation after excluding drop-outs or non-use intervals.
Interpret the Pearl Index
Compare results across contraceptive methods and with perfect-use versus typical-use benchmarks.
Formula
Woman-years = (Number of women × Exposure months) ÷ 12
Pearl Index = (Pregnancies ÷ Woman-years) × 100
Pregnancies per 100 woman-months = (Pregnancies ÷ (Women × Months)) × 100
Full Description
The Pearl Index quantifies contraceptive effectiveness by standardizing pregnancy incidence to 100 woman-years of exposure. It is widely used in clinical trials and comparative studies but can be influenced by sample size, adherence, and differences in exposure time. Perfect-use rates measure efficacy under ideal conditions, while typical-use rates reflect real-world adherence and user error. Interpret Pearl Index alongside continuation rates and user satisfaction for a comprehensive assessment.
Frequently Asked Questions
How does Pearl Index compare to life-table analysis?
Life-table (Kaplan–Meier) analysis accounts for time to pregnancy and drop-outs, offering more precision for long-term studies.
Can Pearl Index be used for small cohorts?
It can, but small sample sizes exaggerate fluctuations. Report confidence intervals when possible.
What is the difference between typical and perfect use?
Perfect use reflects consistent, correct use every time. Typical use includes human error, missed doses, or improper application.
Why adjust for woman-years?
Standardizing to woman-years enables comparison across studies with differing durations and cohort sizes.