$$ Pearl \space index = \frac {Number\space of \space pregnancies \times 12}{(Number\space of \space women) \times (number \space of \space months)} \times 100 $$

Where,

• Total accidental pregnancies: Include every known conception, irrespective of its outcome
• Total months of exposure: Deduct 10 months for full-term deliveries and 4 months for abortion, from the total period under review.