What is the margin for error of an ultrasound at 15 weeks? Preg confirmed at about 11 wks, wouldn't US have been done then?

Fetal Ultrasound. An Ultrasound may be done prior to 18-21 weeks gestation if a patient is unsure of her dates or there is a question of whether the pregnancy is within the uterus and not ectopic, i.e. tubal pregnancy or.if the patient is having vaginal bleeding, ultrasound may be performed to confirm whether the fetus is still living. Otherwise, the first ultrasound is usually done at 18-21 weeks.