Pregnant women in the U.S. who want to know whether they're carrying a fetus with Down syndrome now have access to a commercial genetic blood test that has a 99 per cent accuracy rate.

Down syndrome, which results in a range of cognitive delays, is caused by having an extra copy or parts of chromosome 21. 


A blood test to detect Down syndrome builds on the discovery that fetal DNA can be found in a pregnant woman's blood. ((Joe Raedle/Getty))

Current screening tests such as amniocentesis involve inserting a needle into the woman's womb to get a sample for diagnosis. Amniocentesis is invasive and comes with a risk of miscarriage.

But as of this week, Sequenom is offering in 20 major U.S. cities a blood test that analyzes fetal DNA in the woman's blood.

The company published a study this week in the journal Genetics in Medicine that suggested the test picked up 98.6 per cent of fetuses with Down syndrome with a false-positive rate of 0.20 per cent.

The blood test, called MaterniT21, can be used as early as 10 weeks into a pregnancy for those at high risk. 

"The pregnant women can be safely assured that all we will be taking is just a blood sample," said Dr. Rossa Chiu of the Chinese University of Hong Kong, who helped develop the test. "The majority of cases would not need to go on to an invasive test."


The majority of women who get the blood test will not need to go on to an invasive test such as amniocentesis, says Dr. Rossa Chiu. ((CBC))

In a new era of genetic tests for pregnant women, other companies are also working on blood tests for Down syndrome and other conditions. The advances build on the discovery that there is fetal DNA circulating in the woman's blood. 

At a prenatal Pilates class in Toronto, Elena Leontieva said at six months into her pregnancy with her first baby, all signs look good.

If a non-invasive, accurate test was available early in pregnancy for genetic abnormalities, Leontieva said that would be ideal.

Early guidance

"You want to be just sure your baby is developing all right and you kind of want to know at the early stages whether there is a problem," she said.

Up to six per cent of babies are born with some sort of problem, and a substantial proportion of those are potentially amenable to prenatal diagnosis, said Dr. Mark Evans, a professor of obstetrics and gynecology at Mount Sinai Medical Center in New York.

"While prenatal diagnosis conjures up the Machiavellian fears of search and destroy missions, in fact, what we do for most patients is give them good news and reassure them," Evans said.

Prenatal diagnosis doesn't aim to completely replace invasive tests like amniocentesis but to give doctors a better handle on who really needs those procedures, Evans said.

"The goal is to give families and women the best information that they can use for their own individual decisions," said Dr. Douglas Wilson, a medical geneticist at the University of Calgary.

When a woman is known to be carrying a baby with Down syndrome it will change her obstetrical care, Wilson noted.

Sue Robins of Edmonton is raising a child with Down syndrome. She's concerned about the broader implications of an easy blood test for genetic disorders.

"I'd just like people to ask themselves what kind of world do they want to live in? Do they want to live in a world of diversity where there's lots of different kinds of people? Or do they want to live in a really homogenous world?"

So far, pregnant Canadians don't have access to prenatal diagnostic blood tests. But at a recent genetics conference in Montreal, experts said the field is developing very rapidly.

With files from CBC's Kelly Crowe and Heather Evans