My doctor changed my due date, why?

Due date: The estimated due date is usually from the last menstrual period and also from early fetal ultrasound. When there is a large discrepancy with the due date between the two, the doctor may opt to use the due date based on first trimester ultrasound which usually is within 3 to 5days. This usually give the best estimate of due date when the menses is irregular.

Ask him/her: The original date may have come with the last normal menses. or a 1st ultrasound. Revisions are generally done when findings on a later ultrasounds give a better estimate than the previous method.