Calculating 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 as day 1. An estimated due date can be calculated by following steps 1 through 3:
First, determine the first day of your last menstrual period.
Next, count back 3 calendar months from that date.
Lastly, add 1 year and 7 days to that date.
For example: Your last menstrual period began on September 9, 2010. Counting back 3 calendar months would be June 9, 2010. Adding 1 year and 7 days would bring you to June 16, 2011, as your estimated due date.
This 3-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.
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).