Medical professionals calculate your pregnancy due date based on the gestational age of the baby. Gestational age is counted from the date your last period started prior to conception. In "Pregnancy Week by Week," the Mayo Clinic says that the due date is 40 weeks or 280 days after the start of your last period. The actual age of an unborn baby is called the fetal age and is counted from the date of conception.
Step 1
Determine the exact date of conception or make the best estimate you can if you're not sure. Normally, the date of conception is about 14 days following the start of your last period.
Step 2
Mark the date of the start of your last period before conception took place on a calendar. Note the day of the week as well as the month and date. Count this date as day zero. For example, if the date is on a Friday, label this Friday as day zero.
Step 3
Count forward by the day of the week until you get to the 40th week. If your day zero is a Friday, count forward until you get to the Friday 40 weeks from day zero. Look at the month and date of the 40th Friday. This is your child's due date.
Tips and Warnings
- Due date is always an estimate. Because of individual differences, it's normal for a baby to be born anywhere from 10 days before to 10 days after the due date.
Things You'll Need
- Calendar
