How to Compute Pregnancy Due Date

Calculating the baby's due date is one of the most exciting moments for expecting parents. In humans, pregnancy lasts about 38 weeks from conception, or 40 weeks from the last menstrual period (LMP). According to the World Health Organization, normal gestation may last 35 to 40 weeks from conception, or 37 to 42 weeks from the LMP.

When estimating the day of birth, it is important for mothers and fathers to realize that due date formulas are only estimates, and should not be interpreted as exact dates. On average, only 5% of human babies are born on the exact day of their predicted due date. Roughly 67% of babies are born with in +/- 10 days of their expected due date.

The most common formula for calculating the day of birth is counting 280 days (40 weeks) from the last menstrual period. This assumes that human gestation lasts 266 days (38 weeks) from conception and that conception occurs about 14 days after the last period. (266 + 14 = 280). For women whose cycles are longer than 28 days, this formula is slightly less accurate. For example, if a woman menstruates every 30 days, she would have about 15 days between her last period and conception.

Another method is to count 266 days from the date of conception, if that date is known.

The calculator on the left computes the date that falls exactly 280 days after any date you enter. It accounts for leap years and months with more than 30 days.

If you don't have access to this calculator, you can estimate the due date by hand using Naegele's Rule. Just add nine months and a week from the date of the last period. For example, if the last period occurred on March 1 2008, then the baby's expected day of birth is around December 8. (If you use the calculator, you will find that December 6 is exactly 280 days from March 1.)

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