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Ultrasound: the Most Important Prenatal Test

For roughly 70 percent of all pregnant women, the first picture of their baby is a blurry ultrasound image. Most doctors use this sonar-based technology regularly, and a growing number of insurance plans cover at least one ultrasound test during pregnancy.

But what can a scan really tell you -- and your doctor -- about the baby? "A lot," says Frank Chervenak, M.D., director of fetal medicine at Cornell Medical Center, in New York City. Experts agree ultrasound can be an important tool for diagnosing conditions throughout pregnancy, from the causes of unexplained maternal bleeding during the first trimester to the levels of amniotic fluid in the last.

Because of the size and development of the fetus, most doctors prefer to do a routine scan during a woman's second trimester, usually at 18 to 20 weeks. This is a good time to gauge gestational age and growth, and to check for multiple fetuses. The doctor should also survey your baby's anatomy to try to ascertain any problems.

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.

Who's Looking at You, Baby

Ultimately, the more experienced the technician and doctor, the better chance you have of getting an accurate ultrasound. To help ensure a high-quality one: First, make sure the facility you use is accredited by the American Institute of Ultrasound in Medicine or the American College of Radiology. Ask your doctor who will be reading the sonogram. "The fact that we have sonographers is helpful," says Dr. Benacerraf. "But they shouldn't be responsible for reading scans. A physician should be around." So if a technician is performing the ultrasound, chances are that a doctor will interpret it later, but make sure by asking first.

If you can't find a private practice that's accredited for ultrasound, consider having your sonogram done at a major medical center or a teaching hospital, if you live near one. The doctors there will probably have more experience, and as a result may be better able to detect birth defects than those with a less practiced eye.

The Controversy

Some experts wonder whether the cost of an ultrasound procedure -- approximately $200 -- makes it worthwhile as a screening test, since they say routine scans don't have much impact on the outcome of a pregnancy. If a baby has a birth defect, there is probably little that can be done to correct the problem until after the delivery.

But others maintain that ultrasound has many benefits. For most women, that first glimpse helps them bond with their baby-to-be. And if a sonogram uncovers something wrong, expectant parents can prepare themselves -- emotionally and medically -- to deal with their baby's condition.

Also, these experts say, the detection rate is pretty good. "In general, major birth defects occur in two to three percent of all pregnancies," says Dr. Kuhlman. "If you can find half of these defects, this test should be offered."