If successful, the test could replace the current invasive methods of amniocentesis and chorionic villus sampling, which are associated with an increased risk of miscarriage.
The test compares the number of single-nucleotide polymorphism (SNPs) sites between alleles on chromosomes 21 and 13.
Chromosome 21 was selected because infants with Down's carry three rather than two copies of this chromosome.
Chromosome 13 is not associated with Down's and served as a reference.
The study involved 60 women aged between 18 and 43 who were eight to 35 weeks pregnant. Blood samples were collected and the plasma and buffy coat were isolated for analysis.
Polymerase chain reaction was used to amplify the SNPs from the maternal blood. Only specific SNPs that showed a unique foetal allele in the maternal plasma were quantified. For each of the 60 samples, 570 SNPs were analysed on chromosome 21 and 549 SNPs were analysed on chromosome 13.
The number of SNPs on chromosomes 21 and 13 that were maternal or foetal were counted and the ratio calculated.
This ratio allowed the researchers to determine if the foetus was carrying extra copies of key chromosomes. The results were then compared with amniocentesis or newborn reports.
The test was correct for 58 out of the 60 samples analysed. The test correctly identified 56 of the 57 normal samples and two of the three trisomy 21 samples.