On average a woman with a normal 40 week pregnancy will have 3 ultrasounds. In the case of a pregnancy with twins or multiples as many as 10 or more exams may be performed. A complicated pregnancy can also warrant 10 or more ultrasounds over the duration of the pregnancy.
An ultrasound can be performed in the first trimester. Keeping in mind the fetal heart rate and fetalpole (tiny fetus) cannot normally be visualized before 5.5 weeks at the earliest.