When during pregnancy will I have my first ultrasound?

First trimester. In most cases you will have an ultrasound in your first trimester to confirm the viability of the pregnancy and make sure there is just one fetus.
8-10 weeks. In most practices that I am aware of, an ultrasound is performed early in order to establish the best dating criteria for the pregnancy and give a firm due date. Later ultrasounds are better for evaluating the fetal anatomy (usually 18-20 weeks) but they are less accurate at establishing a due date.