It is common for each ultrasound throughout pregnancy to predict a different due date. Earlier ultrasounds are more accurate in terms of predicting the due date, so thats why doctors will usually use the dates and measurements from the first ultrasound of the pregnancy as a reference.

What does the date on an ultrasound mean?

A dating ultrasound or dating scan is a scan done early in pregnancy (before 12 weeks). It is usually done to check how many weeks pregnant you are and therefore to calculate the estimated due date, but it can be done for other reasons. •

How can you tell how old an ultrasound is?

If there is more than one first-trimester scan with a mean sac diameter or crown-rump length measurement, the earliest ultrasound with a crown-rump length equivalent to at least 7 weeks (or 10 mm) should be used to determine the gestational age.

How accurate are due dates?

But data from the Perinatal Institute, a non-profit organisation, shows that an estimated date of delivery is rarely accurate - in fact, a baby is born on its predicted due date just 4% of the time.