How to calculate a due date
A typical pregnancy lasts, on average, 280 days, or 40 weeks--starting with the first day of the last normal menstrual period (LMP) as day 1. An estimated due date (EDD) can be calculated by following steps 1 through 3:
First, determine the first day of your last menstrual period.
Next, count back three calendar months from that date.
Lastly, add one year and seven days to that date.
For example: Your last menstrual period began on September 9, 2010. Counting back three calendar months would be June 9, 2010. Adding one year and seven days would bring you to June 16, 2011, as your estimated due date (EDD).
This three-step method is referred to as Naegele's Rule and is based on a normal 28-day menstrual cycle. Therefore, dates may have to be adjusted for longer or shorter menstrual cycles. However, even with adjustments made, less than 10 percent of women actually give birth on their calculated due dates. Delivery two weeks before or after the estimated due date is still considered normal.
You may also estimate your delivery date by using the steps 1 and 2 and the chart:
First, locate the first day of your last menstrual period (in black text).
Next, note the date directly below. This is your estimated date of delivery (in blue text).