If both estimates are within a few days of each other, we can use either the LMP or the ultrasound estimate for the due date. However, if they are quite different, the ultrasound due date is more reliable. A dating ultrasound or dating scan is a scan done early in pregnancy (before 12 weeks).

Is a dating scan an ultrasound?

A dating scan is an ultrasound scan to determine how many weeks pregnant you are and your due date.