The scan or my dates - which should I believe? Usually the expected date of delivery (EDD or EDC) is calculated from your last menstrual period - if the early dating scan calculates the EDD to be within 5 days of the EDD from your last menstrual period.

How do doctors determine due date by ultrasound?

An ultrasound allows your doctor to measure the crown-rump length (CRL) — the length of the fetus from one end to the other. During the first trimester, this measurement provides the most accurate estimation for the age of the baby. Your doctor may change your due date based on the ultrasound measurement.

How many weeks of pregnancy the ultrasound dating scan is carried out?

If youre pregnant in England youll be offered an ultrasound scan at around 10 to 14 weeks of pregnancy. This is called the dating scan. Its used to see how far along in your pregnancy you are and check your babys development. The scan may also be part of a screening test for Downs syndrome.