Note: When adding the 7 days, remember to account for the number of days in a month (28 vs 30 vs 31).  The image to the right shows a way to remember which months have 30 versus 31 days

• Confirmation of Pregnancy and Due Date:  The confirmation of pregnancy and due date is done by ultrasound. This ultrasound is often done between eight to ten weeks and confirms that the fetus has a heartbeat, is located in the uterus, and the size matches how big it is compared to what it should be. If this ultrasound matches the date within five days of what you expect, the EDD stays the same. If it measures outside the five days, the EDD would be changed to the date determined by the ultrasound.  The ultrasound is most accurate in the first trimester and is less accurate as the pregnancy progresses.  As the pregnancy progresses, an ultrasound done around 20 weeks will have a discrepancy of up to 10 days between the EDD and the measurements of the fetus.  Any estimation of due date done in the third trimester will can have a difference of up to 21 days between measurements.