In pregnancy, a due date is the estimated date of delivery for your baby. Your doctor normally determines your due date based on the first day of your last period and the approximate size of your uterus. If you know your expected due date, you can make a simple mathematical calculation to determine the number of weeks from your expected due date to another date.
Estimate your expected due date. Determine the start date of your last normal menstrual period. Add seven days to this date. For example, if the first date of your last period was December 1, add seven days to this, to get December 8. Count back three months. Continuing the same example, you estimated due date is September 8.
Determine the number of days. To do this, you can either use a calendar and count the days, or you can use an online tool. For example, assume there are 150 days until your expected due date.
Divide the number of days figure from Step 2 by 7. Continuing the same example, 150 / 7 = 21.43.
Round the week figure from Step 2, to the nearest whole number. Continuing the same example, you would round down from 21.43 to 21. This figure represents the approximate number of weeks from your expected due date.
A doctor can estimate your expected due date more accuracy with an ultrasound. Estimating your due date based on your last menstrual period is generally considered accurate within two weeks.