They are typically performed at 18-20 weeks of pregnancy. Comprehensive exams provide a more in-depth look at the fetus when there is reason to suspect something is wrong with the fetus or mother. They include a detailed examination that is often done as a response to an abnormal screening test such as the alpha-fetoprotein screen. In some areas of the country this is called a Level II or Level III ultrasound. Technical difficulties and the need to image many different areas of the fetus may extend the length of this exam to 30 or more minutes.
Controversy remains regarding the value of the routine prenatal ultrasound for all pregnant women.
Many researchers have studied the benefit of routine ultrasounds for every pregnant woman. Some studies have suggest a benefit, and others do not. This contradictory evidence has resulted in conflicting recommendations from different physician groups. Prenatal ultrasounds are routinely recommended and offered to all pregnant patients in some European countries and in Canada. Currently between 45 percent and 70 percent of women have an ultrasound some time during their pregnancies.