New Test for Down Syndrome

One of the most common severe birth defects, Down syndrome (DS), occurs in about 1 in 800 live births. Often recognized by characteristic facial features, babies born with DS have other abnormalities, including moderate to severe learning disabilities with and average IQ of 40; short stature; visual and hearing problems and/or heart defects; increased risk of infections; and shortened lifespan. Now, analysis of DNA in the blood of the expecting mother can identify a DS fetus.

Diagnosis of DS

DS is caused by a specific chromosomal defect called trisomy 21. A certain diagnosis of DS can be made by detecting trisomy 21in fetal genetic tissue by using one or two tests: chorionic villus sampling or amniocentesis. The former test, which involves obtaining tissue from a chorionic villus of the placenta, usually takes place at 10 to 12 weeks of pregnancy. Amniocentesis, which is done later in the pregnancy, removes a small amount of amniotic fluid from the amniotic sac surrounding the developing fetus. Both of these invasive procedures are costly, and more important carry a small but significant risk of damaging the fetus or causing miscarriage. Accordingly, these tests are generally carried out only when screening tests indicate that a pregnant woman is at high risk for having a DS offspring.

Typical screening tests for DS

Because Down syndrome is more common with advancing age of the mother, genetic counseling and diagnostic tests previously were reserved for pregnant women who were 35 or older. However, the American College of Obstetricians and Gynecologists now recommends that women of all ages should have screening tests for Down syndrome before the 20^th week of pregnancy.

Having said this, the prevalence of DS does increase with the age of the mother. The prevalence is about 1 in 1,500 in young women and rises slowly until age 35; but between ages 35 and 40 the prevalence increases about 4-fold. Women over the age of 35 account for about half of DS births in the U.S.

The screening tests are not invasive. They involve measuring blood levels of various proteins and hormones as well as carrying out an ultrasound examination of the fetus. Although no further testing is done when the results of the screening tests are negative, a negative screening test does not completely exclude a DS fetus. Positive tests are most often followed by a normal pregnancy, but they naturally create considerable stress and uncertainty. Since invasive tests are usually recommended when screening tests are positive, decisions are best made after meeting with a genetic counselor. 

A new noninvasive screening test

This new test takes advantage of the fact that the blood of pregnant women contains DNA from the fetus. And three small studies showed that it was feasible to identify a fetus with trisomy 21 by analyzing the DNA in maternal blood. In a study of more than 1,100 high risk pregnancies, reported in a January 2011 issue of the British Medical Journal, sophisticated analysis of maternal blood DNA was able to diagnose the presence or absence of a fetus with DS with almost total certainty.  

Why get screened for DS anyway?

Because of the multiple ill effects of DS, a positive diagnosis most often leads to termination of the pregnancy. Those who are opposed to abortion may be helped in their planning for a DS child by meeting with a counselor or by contacting the National Association for Down Syndrome.