Down's syndrome, caused by an extra copy of chromosome 21, is generally detected by amniocentesis, by inserting a needle into a pregnant woman's belly to sample cells from the fluid bathing her fetus. It carries a 1% risk of miscarriage and is usually performed between 15-20 weeks into pregnancy. Chorionic villus sampling, which samples tissue from the placenta, can be performed earlier, but has an even higher risk.

The new tests instead detect DNA from fetal cells that have broken down. Some of this DNA crosses the placenta and gets into the mother's bloodstream, and the tests look for an excess of material from fetal chromosome 21.

First out of the gate was Sequenom of San Diego, which launched its test for Down's last October. Verinata of Redwood City released its test in March, while Ariosa Diagnostics of San Jose will launch a test this week. Later this year, Natera of San Carlos will market a test that can also detect additional copies of the X and Y sex chromosomes.

Not only do the blood tests eliminate the risk of miscarriage, they are also claimed to have lower error rates than conventional tests. Ariosa said that its test turns up false positives in less than 0.1% of cases - compared with 5% from invasive tests. The other companies report similar results. The new tests can be run from about 10 weeks into pregnancy.