Experimental Prenatal Test Helps Spot Birth Defects

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The reason for their rarity? The mother's cells are very much like water balloons: From the moment blood is drawn, her cells burst and let loose maternal DNA into the blood stream. This serves to dilute the fetal DNA, Dhallan explained.

"Every time a mother's cell bursts, it dilutes the fetal DNA, particularly when you transport samples and when you centrifuge and process samples to get the DNA out of the plasma," he said.

In March 2004, Dhallan solved that problem in a study that showed that adding formaldehyde to the blood sample caused the mother's cells to harden ("like Ping-Pong balls"), reducing dilution and increasing fetal DNA to 25 percent.

The next challenge was how to distinguish maternal DNA from fetal DNA and how to identify abnormalities.

For the new study, the researchers examined blood samples from 60 pregnant women and the "stated biological fathers," and analyzed single nucleotide polymorphisms (SNPs or "snips"), the tiny variations in the DNA sequence that exist between individuals.

There are about 3 million variable sites where human DNA differs from person to person, out of a total of 3 billion base pairs. "The key is finding where they differ," Dhallan said.

Combining mathematics and genetics technology, the researchers determined the ratio (between mother and child) of SNPs on different chromosomes. That ratio should be the same for all chromosomes. "If it's not, you have a problem," Dhallan said.

The method correctly identified the number of chromosomes in 58 of 60 samples, including two cases of trisomy 21 (which causes Down syndrome), Dhallan said. One case of trisomy 21 went undetected, while one normal sample was incorrectly identified as trisomy 21.

Katz said: "They have one mistake one way and one the other way. That's a little too risky if you compare this with invasive techniques where you have virtually 100 percent accuracy. Also, one can never be absolutely certain that this is the biological father. In addition, there may be some times when the father is not available."