In January, the American College of Obstetricians and Gynecologists recommended that all pregnant women have access to Down's syndrome screening, which in the past was advised only for women age 35 and older. But prenatal tests for Down's syndrome and other genetic abnormalities can be problematic. Initial screening can be done with ultrasound or a blood test, but a definitive diagnosis requires more-invasive procedures: amniocentesis or chorionic villus sampling, both of which carry a small risk of causing miscarriage. What's more, the screening tests have a 5 percent false-positive rate, which results in many unnecessary procedures.

Now researchers at a Columbia, MD-based biotech company are reporting encouraging preliminary results using a new experimental method for detecting Down's syndrome. The blood test takes advantage of small pieces of fetal DNA floating in a pregnant woman's bloodstream. The development of the method, reported in the current issue of Lancet, is still in early stages, but this represents a first step toward a long-sought goal of a quick, safe, and effective way to perform prenatal genetic tests.

In Down's syndrome, each cell has an extra copy of chromosome 21. To detect this defect, scientists at Ravgen examined fetal-DNA fragments in the blood of pregnant women and looked for a disproportionate level of DNA from chromosome 21. In a study on 60 patients, the research group was able to identify the number of chromosomes correctly in 58 cases. Three patients carried a fetus with Down's syndrome. The test was able to detect two of the three. There was also one false positive among the 57 samples without the extra chromosome 21.

Farideh Bischoff, a molecular cytogeneticist at Baylor College of Medicine, says that the study provides "proof of concept" for the technique. "Conceptually, it all makes sense; scientifically, there's a lot of development to be done." In particular, the sensitivity and accuracy of the test must be improved before it can function as a diagnostic test, and it must be tested on a larger scale.

Several years ago, researchers discovered that a mother's blood contains cells from her baby, as well as fragments of fetal DNA. But Bischoff says that one of the major challenges in the field is distinguishing a mother's DNA from her baby's. Sorting through the scattered DNA fragments is like listening to a radio station playing two channels at once; the trick is to separate the two signals.