Ultrasound: the Most Important Prenatal Test
Screening Put to the Test
As with all prenatal testing, ultrasound isn't 100 percent accurate. "An ultrasound is just a partial view of the fetus," says Dr. Chervenak. "It's not perfect: You don't get as clear a picture as you would in a photograph."
Some abnormalities are simply too small to see, for instance, or the fetus has shifted into a position that obscures the view. And some defects are caused by malfunctioning organs or chromosomes, which an ultrasound may not detect.
"We don't find everything, but we pick up some of the most serious birth defects," says Kathleen Kuhlman, M.D., a specialist in maternal-fetal medicine at Thomas Jefferson University, in Philadelphia. For example, the rates for detecting neural-tube defects, such as anencephaly and spina bifida, are extremely high -- over 90 percent. And many of the more severe heart problems can be caught as well. In fact, the estimated rate for detecting all birth defects with this test is 50 percent.
On rare occasions, a doctor will find in an ultrasound image a birth defect that isn't really there -- a clubfoot or cleft lip, for instance. "Overdiagnosis is the downside to a screening ultrasound, and it can be a source of incredible worry and anguish," says Beryl Benacerraf, M.D., director of obstetrical ultrasound at Massachusetts General Hospital, in Boston.
Fortunately, there are checks in place. If your sonogram has picked up something wrong, your doctor will recommend you have another one. Amniocentesis can also rule out possible genetic defects.