Testing for Down Syndrome Early

Trisomy blood test for Down syndrome and others works at ten weeks of pregnancy

/ Author:  / Reviewed by: Joseph V. Madia, MD

(RxWiki News) One decision pregnant women must make is how much to test for possible disorders in their baby. More tests with fewer risks are becoming available for conditions like Down syndrome.

A recent study found that a blood test at 10 weeks of pregnancy could effectively test for three chromosomal disorders.

These are Down syndrome, Edward syndrome and Patau syndrome. All involve having an extra chromosome.

This new blood test only returned 0.1 percent false positives. This means only 0.1 percent of those tested had a positive result for a trisomy disorder that was not correct.

"Discuss prenatal testing with your OB/GYN or midwife."

The study, led by M. M. Gil, of the Harris Birthright Research Centre for Fetal Medicine at King's College Hospital in London, England, looked at whether a specific blood test at 10 weeks of pregnancy might identify babies with trisomy disorders.

Trisomy disorders occur when a child is born with an extra chromosome. The trisomy disorders studied in this research were trisomy 21, trisomy 18 and trisomy 13.

Trisomy 21 is the disorder that causes Down syndrome. Trisomy 18 is known as Edward syndrome, and trisomy 13 is known as Patau syndrome.

The researchers used a blood test with something called cell-free DNA testing to look for trisomy disorders in 1,005 women, aged 20 to 49, who were pregnant with one child.

Half the women were over age 37 and half were under age 37. Trisomy disorders are more common among older women, especially over age 35.

Initially, 95 percent of the lab results (957 women) were able to be read, nearly all within two weeks of the blood test.

Although 48 women's lab results either did not test properly or had delivery problems at the lab, the researchers retested 40 of the women and got results for 27 of them.

The researchers identified 11 women with a risk score over 99 percent for Down syndrome.

Five women tested as having over 99 percent risk for a baby with Edward syndrome (trisomy 18), and one woman's risk was 34 percent for a baby with Patau syndrome (trisomy 13).

The other 968 women with readable test results had a risk below 0.01 percent for all three trisomy disorders.

The researchers then used a different test process called chorionic villus sampling, which require getting a sample of the placenta tissue to test. This test confirmed all the blood test results except one of the trisomy 18 results.

The researchers also reported the false positive rate for both these tests. A false positive occurs when a test result inaccurately comes back as positive for a disorder or condition when the individual doesn't have that condition.

False positive results occurred in 0.1 percent of the cell-free DNA blood test and in 3.4 percent of the chorionic villus sampling test.

The researchers concluded that a blood test at 10 weeks could effectively screen for Down syndrome and trisomies 13 and 18 with a lower false positive rate than the placental sampling test.

The study was published June 7 in the journal Ultrasound in Obstetrics & Gynecology. The research was funded by the Fetal Medicine Foundation. No disclosures were reported.

Reviewed by: 
Review Date: 
June 6, 2013
Last Updated:
August 7, 2013