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Less-Invasive Blood Test May Be Better at Detecting Down's Syndrome in the Womb

Update Date: Jun 07, 2013 01:58 PM EDT

A simple blood test during the first trimester may soon be able to diagnose unborn babies with Down's syndrome, according to a new study.

Expectant mothers who want to know if their babies have the genetic disease must undergo invasive procedures like an amniocentesis, which comes with serious risks for the mother and baby.  However, the latest findings, published in the journal Ultrasound in Obstetrics & Gynecology the first trimester blood test is superior to currently available non-invasive methods of screening for Down's syndrome.

Down syndrome is a genetic disorder that causes lifelong mental retardation, developmental delays and other problems. And because the condition varies in severity, developmental problems can range from moderate to serious.  The disorder is the most common genetic cause for learning disabilities in children, according to the Mayo Clinic.

In 90 percent of cases, Down's syndrome is caused by a third copy (trisomy) for the 21st chromosome.  Healthy people have two copies of 23 chromosomes, one copy from the father and the other from the mother. In the majority of cases, Down's syndrome is not inherited, but caused by a glitch in cell division during development.

The disorder can cause abnormal facial features, poor muscle tone and susceptibility for conditions like heart defects, infectious disease, dementia, sleep apnea, obesity, vision problems, hearing loss and seizures.

Currently, pregnant women must undergo a "combined test" between the 11th and 13th week of pregnancy.  The test involves an ultrasound scan and a hormonal analysis of the pregnant woman's blood, and can only give a woman an estimate of the chance of her baby having Down's syndrome.

Researchers said there are only two tests that can definitively detect or rule out fetal genetic abnormalities like Down's syndrome. One test is called chorionic villus sampling, which samples a small piece of the placenta, and the other is called amniocentesis, which tests the amniotic fluid around the baby. However, both these tests pose serious risks for the mother and baby and can lead to miscarriage.

While previous studies have already shown that non-invasive prenatal diagnosis for trisomy syndromes using fetal cell free (cf) DNA from a pregnant woman's blood is highly sensitive and specific, the new study is the first to examine the feasibility of using cfDNA to test for trisomy 21 and other chromosomal disorders.

Lead researcher Kypros Nicolaides of King's College London and his team tested 1,005 pregnancies at 10 weeks and found that the cfDNA test had a lower false positive rate and higher sensitivity for fetal trisomy than the combined test done at 12 weeks.

Results from the study revealed that the rate of false positives using the cfDNA test was 0.1 percent compared to 3.4 percent of the combined test.

"This study has shown that the main advantage of cfDNA testing, compared with the combined test, is the substantial reduction in false positive rate. Another major advantage of cfDNA testing is the reporting of results as very high or very low risk, which makes it easier for parents to decide in favor of or against invasive testing," the authors wrote in the study.

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