The earlier the ultrasound is done, the more accurate it is at estimating the babys due date. Ultrasounds performed during the first 12 weeks of pregnancy are generally within 3 - 5 days of accuracy. The most accurate time is between 8 and 11 weeks gestation.

When should I book my dating ultrasound?

Dating ultrasounds are most accurate when they happen between 7-12 weeks, and are calculated by measuring the fetus crown-rump length. This generally predicts the expected date of birth within 5 days.