By Karla Gale
Ultrasound, which carries little or no risks to the mother, can be used to determine the likelihood that a foetus with Down syndrome is present. To know for sure, however, amniocentesis or a related procedure called chronic villus sampling is required.
With amniocentesis, a long needle is inserted through the mother's abdomen into the amniotic sac surrounding the foetus. A small amount of the amniotic fluid is then removed and the DNA in the fluid is analysed to determine if an extra chromosome 21 - the hallmark of Down syndrome - is present. Unfortunately, about one in 200 amniocentesis procedures will cause miscarriage.
"My wife (and our kids!) were subjected to invasive testing and that sensitised me to the risks involved in current procedures," senior author Dr. Stephen R. Quake told Reuters Health. "When I learned that foetal DNA circulates in the mother's blood, I realised I could use (special tests) to measure and count individual molecules."
To diagnose Down syndrome with amniocentesis, "you just have to count three copies of chromosome 21 instead of two," the bioengineer explained. "With non-invasive sampling the counting problem gets a lot more subtle - there is just a small over-representation of chromosome 21 relative to (other) chromosomes. So we used new sequencing technologies to sequence millions of molecules."
Quake and colleagues compared their approach using plasma obtained from 18 women who were 10 to 35 weeks pregnant at the same time they underwent amniocentesis or chorionic villus sampling.
According to the team's report in the Proceedings of the National Academy of Sciences, the new technology successfully identified all nine cases of Down syndrome, two cases of Edward syndrome (three copies of chromosome 18), and one case of Patau syndrome (three copies of chromosome 13).
Quake believes that abnormal numbers of chromosomes in the foetus could be detected as early as the fifth week of gestation, "but this remains to be proved".
In their paper, the researchers note that "the costs of the (test) are already fairly low; the sequencing cost per sample is approximately $700 (about R6 300), and the cost of sequencing is expected to continue to drop dramatically in the near future."