New non-invasive prenatal blood test for Down's syndrome, safer for the fetus

American researchers have created a prenatal blood test which can be used to determine if an unborn baby has Down's syndrome and the new test is safer for the fetus.

Current tests which are considered the "gold standard" in prenatal testing, involve invasive testing methods such as amniocentesis and chorionic villus sampling (CVS), which are obtained by inserting a needle into the mother's uterus in order to obtain sample cells in the amniotic fluid.

The procedure poses a small risk of inducing a miscarriage and routine use of these tests has largely been limited to women age 35 and older, where the risk of fetal abnormalties outweighs the risk of miscarriage and it takes two or three weeks following the procedures to culture the cells and study the chromosomes.

As a result, more infants with chromosomal defects are now being born to younger women and scientists have been searching for years for noninvasive screening tests that would be safe for that group of women.

The researchers at Howard Hughes Medical Institute led by bioengineer Stephen Quake of Stanford University in California have created a way to search for the extra chromosomes that cause Down's and similar birth detects in the tiny amounts of fetal DNA that circulate in the mother's blood.

The test requires a small blood sample from the mother, is much safer than amniocentesis and can be done earlier in a pregnancy.

Down's syndrome, which causes mental retardation and other problems, is caused when a child has three copies instead of the normal two of chromosome 21.

The test also detects other chromosomal conditions such as Edward syndrome, which kills half of babies in the first week of life and Patau syndrome, which kills more than 80% of children in infancy.

The accuracy of the new genetic test was trialled in a small study involving 18 women and it accurately identified the nine women with a Down's syndrome pregnancy and three others with fetuses with different chromosomal disorders.

The researchers say it is the first universal, non-invasive test for Down syndrome and should put an end to invasive testing procedures.

Quake says blood tests for pregnant women such as the alpha-fetoprotein test serve as only ' indirect and weak predictors' of what is going on as while they can detect potential signs of a chromosomal disorder such as Down's, they cannot diagnose it with certainty.

A definitive determination requires amniocentesis which is done typically after the 15th week of pregnancy but the new test can be administered much earlier in a pregnancy - potentially as early as five weeks after conception - and the results are back within a couple of days.

Quake says the earlier a couple know the status of the pregnancy, the better they will be prepared for whatever decision they choose to make on the basis of the information.

Quake says a larger study involving hundreds of women must now be done and then the new test could be widely available in two or three years time and could become a routine prenatal test of a baby's health.

The research is published in the journal Proceedings of the National Academy of Sciences.