The pregnancy dating scan is performed around week 10-11 of the pregnancy. This ultrasound will determine how far along the pregnancy is and it determines the due date. This is done by doing a CRL-measurement (Crown Rump Length).
Please keep in mind that an ultrasound this early in pregnancy is not always possible via abdominal ultrasound. We will always try to perform it abdominally, but sometimes it will be necessary to perform a vaginal ultrasound.