Most dating scans are done with a trans-abdominal transducer and a fullish bladder.
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.
However, the accuracy of the ultrasound examination is always dependent on the skill of the sonographer and the quality of the equipment.
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.