Researchers at King’s College London have identified a new test of fetal DNA in maternal blood that is more accurate and can be administered earlier in the pregnancy.

The study, published in Ultrasound in Obstetrics and Gynecology, examined approximately 1,000 pregnancies with a median maternal age of 37.  The test uses maternal blood for DNA-testing on cell-free content in that blood. The cell-free DNA is the fetal DNA. The test results had a similar distribution of risk for Down’s syndrome as the general population adjusted for risk by average age of mothers. This test enhanced accuracy by showing a lower rate of false positives.

The Mayo Clinic defines Down’s syndrome as “a genetic disorder that causes lifelong mental retardation, developmental delays, and other problems.”

There is a range of severity of Down’s syndrome, but this condition is the most common cause of children’s learning disabilities.  Down’s syndrome is caused by a mutation at chromosome 21.  The most common form is trisomy 21, in which there is an extra chromosome.  The less common version is called mosaic Down’s syndrome, and only some cells have an extra copy of chromosome 21.  The least common version is translocation Down’s syndrome, where part of chromosome 21 becomes attached to another location.  Translocation Down’s syndrome is the only type that a child can inherit from the parents.

As a routine part of prenatal care, screening for Down’s syndrome is available.  The most common test currently is the combined test during week 11 to 13 of the pregnancy.  This test includes an ultrasound to measure for an abnormality on the back of the baby’s neck and a blood test.  More extensive testing uses additional screening during the second trimester.

If prenatal screening is not available or the expectant parents choose to forgo it, doctors may be able to identify Down’s syndrome from physical characteristics after birth.  These include low muscle tone, eyes with an upward slant, appearance of a flattened face when looking at the profile of the baby, a small nose, and excessive space between the big toe and the second toe.  The doctor can also perform a karyotype test to confirm accurately the presence or absence of Down’s syndrome.

One in every 691 live births is estimated to be a Down’s syndrome baby.  This is an increase from one in 1,087 in 1990.  Following a prenatal diagnosis of Down’s syndrome, approximately 67 percent of pregnant women choose to terminate the pregnancy.  This is a marked decrease from 90 percent terminating pregnancies in the mid- to late 1990s.