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."
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.
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