Until now, maternal blood tests have only been able to detect hormone markers that may show an increased risk for Down syndrome. Those who are determined to be at higher risk are advised to undergo CVS or an amniocentesis for a definitive diagnosis.
Both invasive procedures carry a 1 percent risk of miscarriage.
In 1997, Dr. Dennis Lo and his co-researchers at CHUHK discovered that during pregnancy a fetus releases its DNA into the mother's blood plasma. Now they know that the entire fetal genome is present in the mother's blood.
Down syndrome is the most common genetic condition in the United States, causing an array of physical and mental challenges for both child and parents.
Children with the genetic disorder, also known as trisomy 21, have three copies of chromosome 21.
With new sequencing technology, scientists can analyze millions of DNA fragments from each blood sample, looking for abnormalities of chromosome 21.
For decades, amniocentesis has been the gold standard of prenatal testing, performed on 430,000 to 600,000 women a year. The procedure is usually performed at 18 weeks gestation when termination can be traumatic. Doctors insert a needle into the uterus and withdraw amniotic fluid, which can cause fetal injury or miscarriage in 1 out of 200 patients.
Chorionic villus sampling -- is also invasive, but can be done earlier -- at 10 to 13 weeks -- and entails getting a sample of placental tissue. It carries a 1 in 100 miscarriage risk and fetal injury.
Nuchal translucency screening, a type of ultrasound that's done at 11 to 13 weeks in conjunction with blood tests, is safe, but yields an unclear diagnosis.
Existing tests can detect 90 to 95 percent of cases of Down syndrome, but have a 5 percent false positive rate. Each year about 40,000 of the 3 million who take the blood test are directed toward amniocentesis for follow-up.
"The ideal is to aim for 100 percent detection," said Chui, but until new technologies are perfected, new blood tests could supplement current screening.
For example, Hong Kong has 40,000 births a year and about 50 children with Down syndrome, according to Chiu. "But we don't know which 50 they are."
If all 40,000 went through the existing ultrasound and hormone blood test protocols, about 2,000 would be flagged as high risk and sent for invasive testing even though "the majority are normal," she said.
And of those, 1 percent, or about 20, would miscarry.
But with the new DNA blood test, only about 40 of those 2,000 women would be labeled "positive" for Down syndrome and .4 percent would miscarry.
"We are saving babies," said Chui. "And just because we know the results doesn't mean women have to terminate. We are saving women from losing normal children just because of a procedure."
Chui said labs need to be upgraded to handle the sophisticated testing and right now the cost -- $2,000 a test -- is prohibitive.
But American geneticists say the science of sequencing is moving so fast that these blood tests should soon be available.