Doctors hopeful about new test for birth defects

November 12, 1992|By Thomas H. Maugh II | Thomas H. Maugh II,Los Angeles Times

In the first large-scale application of a new, non-invasive prenatal test, Tennessee physicians have identified, in fetal cells taken from the mother's blood stream, chromosomal abnormalities of the type that produce Downs' syndrome and other severe birth defects.

The researchers used the technique on 69 pregnant women and identified the abnormalities in seven fetuses.

Researchers have high hopes for the new test, which uses sophisticated new techniques to isolate the extremely rare fetal cells from the mothers' blood.

"It opens up prenatal diagnosis to the whole population because there is no risk to the fetus," said Dr. Sherman Elias of the University of Tennessee, primary author of the paper.

The more conventional prenatal tests -- amniocentesis and chorionic villus sampling -- both pose a small but significant risk to the fetus because a biopsy needle must be inserted into the womb.

For younger women, the risk from these procedures is greater than the risk of having a Downs' baby, so neither test is normally given to pregnant women under age 35.

But 85 percent of Downs' babies are born to women under age 35.

The new technique, described yesterday at a meeting of the American Society of Human Genetics in San Francisco, is so promising that the National Institutes of Health is organizing a nationwide trial in which the new method will be used on 3,000 pregnant women.

"This is not ready for use at your local doctor's office yet, but we are ready to test it in much larger numbers of people," said Dr. Diana W. Bianchi, a medical geneticist at Boston Children's Hospital.

In a separate study reported at the San Francisco meeting, Dr. Wolfgang Holzgreve of the University of Munster in Germany reported that he had used the new technology to detect an abnormality called trisomy 18 in four pregnant women.

Trisomy 18 produces low birth weight, congenital heart disease, severe retardation and a limited life span.

The new technology makes use of the fact that a very small number of fetal cells, particularly red and white blood cells, leak into the mother's blood during the first and second trimesters of pregnancy.

Such cells account for about one in every million cells in the mother's blood. A typical blood sample used for genetic testing, about four teaspoons worth, contains about 200 fetal cells.

The presence of these fetal cells in the mother's blood has been recognized for more than 100 years, but it has only been within the past five that researchers have devised ways to separate the fetal cells from the mother's.

The key is to identify proteins that are present on the surface of the fetal cells but not on the mother's cells.

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