Doppler Ultrasound – This imaging procedure measures slight changes in the frequency of the ultrasound waves as they bounce off moving objects, such as blood cells.
If you have more than one ultrasound during your pregnancy, giving you 'multiple dates', then the earliest ultrasound estimate should be used, because it will be more accurate.
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.
For transvaginal ultrasound you can have an empty bladder Dating scans are usually recommended if there is doubt about the validity of the last menstrual period.
For example if you a tiny sac can be seen in the uterus, but the baby and its heart beat may not be detected yet.