The due date is 40 weeks from the first day of the last menstrual period. Because the first day of the last menstrual period is sometimes not accurate, the first ultrasound will be compared with the due date from the period. A final due date will be determined from this comparison.
Normal pregnancy is 40 weeks when counting form the last menstrual period. Sometimes people wonder why it is 40 weeks since pregnancy is supposed to be 9 months. Conception actually occurs about 2 weeks after the start of the menses, on week 2 of pregnancy. Each month has between 28-31 days so most months are a bit longer than 4 weeks. When you do the math it turns out pregnancy is about 9 months and one week from the last menstrual period or 1 week less than 9 months from conception.
Dating a pregnancy from the last menstrual period assumes normal 28 day cycles, this is not always the case. Ultrasound is used to confirm the accuracy of the due date. The earlier the ultrasound, the more accurate it is for dating a pregnancy. Ultrasounds done in the first trimester have an error of +/- 1 week. Second trimester ultrasounds are +/- 10-14 days. Third trimester ultrasounds have an error of about 3 weeks. So once a due date is established from the last menstrual period and the first ultrasound it will not be changed. If after the first ultrasound a subsequent ultrasound suggests a different due date, this does not change the due date, but rather says something about how the baby is growing.
EDC is the abbreviation doctors use for due date. EDC stands for estimated date of confinement. The EDC is not necessarily the day that the baby will be born. Only about 7% of babies are born on the actual due date. The EDC is really used for accounting purposes, timing of tests, to determine when it is safe for the baby to come and when it is getting too far along.
When all the information coincides the due date is considered to be more reliable. A first trimester ultrasound that agrees with a normal and certain last menstrual period is considered a very reliable due date. Someone with late prenatal care that does not know her last menstrual period and only has a third trimester ultrasound would be an example of an unreliable due date.