Conception Date Calculator — Find When You Conceived from Birth Date

Formula

How this is calculated

Human gestation from fertilisation to birth averages 266 days (38 weeks). The calculator subtracts 266 days from the birth date to estimate the most likely conception date. Because sperm can survive in the reproductive tract for a few days and the exact moment of fertilisation cannot be known without ovulation tracking, a ±2-day window is shown around the estimated date.

The estimated LMP (last menstrual period) is found by subtracting a further 14 days from the estimated conception date. In a standard 28-day cycle, ovulation — and therefore conception — falls approximately 14 days after the LMP. Together, conception at 14 days plus 266 days of gestation gives the familiar 280-day (40-week) figure used by Naegele's rule for due-date estimation.

This estimate assumes a full-term birth at exactly 266 days post-conception. Premature or post-term births shift the result proportionally — for example, a baby born two weeks early would move the estimated conception date two weeks later than shown. The estimate also assumes a 28-day cycle with day-14 ovulation; shorter or longer cycles shift the LMP estimate accordingly. Only a confirmed ovulation or IVF date provides a more precise answer.