Ultrasound is not consistently able to confirm viability and fetal heart activity before 6 weeks gestation, so unless there is an urgent need to have a scan earlier, most of these ultrasounds are performed after 6 weeks and ideally at 8 weeks for dating purposes.

How accurate are dating ultrasounds at 6 weeks?

Up to and including 13 6/7 weeks of gestation, gestational age assessment based on measurement of the crown–rump length (CRL) has an accuracy of ±5–7 days 11 12 13 14. Measurements of the CRL are more accurate the earlier in the first trimester that ultrasonography is performed 11 15 16 17 18.