How this calculator works
The Pearl index is an incidence rate, not the probability that a particular person becomes pregnant in one year. Use actual eligible exposure under the study protocol, accounting for dropout and excluded cycles. Equal-follow-up mode is appropriate only when every participant contributes the stated months. The 28-day-cycle convention uses 13 cycles per study year and therefore a factor of 1300; calendar-month mode uses 1200. Reverse solutions can be fractional expected quantities. The result is an estimate for information only and is not medical advice.
Formula
Calendar-month basis: PI = 1200 × pregnancies / (participants × months). Cycle basis: PI = 1300 × pregnancies / eligible 28-day participant-cycles. Rearrange to solve the chosen variable.
Worked example
Two pregnancies among 100 participants followed for 12 months gives PI 2. Three pregnancies in 1300 eligible cycles gives PI 3.
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Frequently asked questions
Is PI a percentage?
No. It is pregnancies per 100 woman-years of exposure and should be interpreted as a rate.
Why are there factors 1200 and 1300?
They represent 100 people for 12 calendar months or 13 study cycles of 28 days.
Can I use average follow-up with dropout?
Use total eligible exposure where possible. A simple planned duration can overstate actual follow-up.