Down's Syndrome: Can an Ultrasound Really Detect It?

Frederick R. Jelovsek, MD

One of the risks the older mother has is the increased chance of having a baby with a chromosomal defect. The most common of these anomalies is the baby with Down syndrome (trisomy 21, mongolism). It accounts for about one-half of all the genetic anomalies associated with aging. This is the main reason genetic amniocentesis (drawing fluid from the baby's sac) is recommended to pregnant women.

Many women are afraid to undergo amniocentesis both because fear of pain or of hurting the baby, or because of fear of what the results might show. For quite awhile, physicians have tried to diagnose Down syndrome by ultrasound, looking at subtle findings such as neck skin thickness and slightly shortened long bones. The question becomes "how accurate is ultrasound at picking up Down syndrome."

Diagnosing Down's Syndrome

A recent study looked at additional subtle findings to see if they can improve the accuracy of diagnosis. Deren O, Mahoney MJ, Copel J, Bahdo-Singh RO: Subtle ultrasonographic anomalies: Do they improve the Down syndrome rate? Am J Obstet Gynecol 1998;178:441-5. They found that the diagnosis was improved from about 30% to 63%.

In one respect, that's a tremendous improvement. It helps to pick up the cases of Down syndrome in younger women who would not be identified as at risk due to age. In another respect, it reminds us that ultrasound is not yet a substitute for the genetic amniocentesis. Mothers should not be encouraged to have an ultrasound (called a level 2 scan) instead of the tap. Also physicians need to remind mothers that they will miss at least a third of the instances of Down syndrome by ultrasound alone.