A study published in the latest Lancet raises hopes of prenatal testing, done by analyzing a sample of mother's blood. The technology involves separating fetal DNA and mother's DNA from the sample, and then by looking into extra copies of chromosome 21.

Currently, available tests for prenatal diagnosis of chromosomal abnormalities--eg, trisomy 21, which causes Down's syndrome--are limited by several factors. Screening tests, such as ultrasound, are non-invasive, but diagnosis requires further invasive testing. Invasive diagnostic tests, such as amniocentesis and chorionic villus sampling, are associated with risks to the pregnancy. Development of non-invasive tests that yield diagnostic results would be an important advancement in prenatal care.

Ravinder Dhallan (Ravgen Inc, Columbia, MD, USA) and colleagues took blood samples from 60 pregnant women and extracted fetal DNA from these samples. The investigators examined the samples of fetal DNA for chromosomal abnormalities by analysing an array of single nucleotide polymorphisms (SNPs, pronounced "snips")--tiny variations in the DNA sequence of individuals.

The researchers established the ratio of SNPs on different chromosomes, which enabled them to determine whether the fetus had chromosomal abnormalities. Of the 60 samples tested, the technique identified the number of chromosomes correctly in 58, including two cases of trisomy 21. Although one case of trisomy 21 was not identified, and one normal sample was incorrectly identified as being trisomy 21, the researchers stress that this is a preliminary study, and that further trials are needed to fine tune the technique.