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New Use For Ultrasound

Down Syndrome

Down syndrome is a congenital condition caused by a chromosomal abnormality. Normally, humans have 46 chromosomes (23 pair). Children born with Down syndrome have 47 chromosomes, usually an additional chromosome 21. The extra genetic material in the chromosome leads to the development of characteristic features of Down syndrome and varying degrees of mental retardation.

According the National Down Syndrome Society, the condition is the most common chromosome abnormality. About 350,000 Americans have the condition and roughly 5,000 babies are born with it every year. Risk of having a child with the syndrome increases with maternal age. At 35, a woman has about a one in 400 chance of conceiving a Down syndrome child. By 40, the risk jumps to one in 110, and by 45, it’s one in 35.

Prenatal Screening and Diagnosis

About 97 percent of cases of Down syndrome occur in families with no previous history of the condition. Doctors usually offer a blood test to screen pregnant women for the condition using three markers: serum alpha feto-protein (MSAFP), chorionic gonadotropin (hCG), and unconjugated estriol (uE3). While not a definitive test, generally, lower levels of MSAFP and uE3 along with elevated levels of hCG, are associated with an increased risk for the syndrome. When combined with an ultrasound examination, the blood test is about 60 percent accurate in predicting risk for fetal abnormalities suggestive of Down syndrome.

Confirmation of the condition can be made through one of several prenatal diagnostic tests. Amniocentesis involves the withdrawal of a small amount of the amniotic fluid (the fluid surrounding the baby) through a needle inserted in the abdomen. Chorionic villus sampling is the removal of a small amount of tissue from the placenta (called the chorionic villi). The tissue sample is obtained by inserting a needle into the abdomen or a catheter through the cervix.

Percutaneous umbilical cord sampling (PUBS, also called cordocentesis) is the withdrawal of a small amount of blood from a vein in the umbilical cord. The three diagnostic tests are about 98 percent accurate in their ability to detect chromosome abnormalities associated with Down syndrome, but they carry a small risk of miscarriage.

Nuchal Translucency Screening

Some physicians are offering another screening test for pregnant women, called the nuchal translucency screening. Nuchal translucency refers to an area at the back of the neck in the fetus, where blood and fluids pass into the brain. In a fetus with abnormal heart function or lymph flow, extra fluid pools in the area.

Using ultrasound scans, doctors can measure the nuchal translucency to determine how much fluid is accumulating. Research suggests a larger nuchal translucency is associated with an increased risk for Down syndrome, heart abnormalities, and unexplained pregnancy loss.

The nuchal translucency scan is non-invasive and about 80 percent accurate in detecting abnormalities associated with Down syndrome. The scan is done sooner in the pregnancy than some other screening/diagnostic tests (around 10-14 weeks), so doctors can make an earlier diagnosis.

Since most screenings produce negative results (little or no risk for Down syndrome), the scan can help put a family’s worries to rest. On the other hand, women who have positive results have more time to prepare for a child who may have special needs and medical intervention after birth.

  • For general information on Down Syndrome or prenatal screening and diagnosis: Medem™ - www.medem.com
  • National Down Syndrome Congress
    7000 Peachtree-Dunwoody Rd., NE
    Lake Ridge 400 Office Park, Building #5, Suite 100
    Atlanta, GA 30328
    www.ndsccenter.org
  • National Down Syndrome Society
    666 Broadway
    New York, NY 10012
    www.ndss.org
    (212) 460-9330
  • National Institute of Child Health and Human Development
    Information Clearinghouse
    PO Box 3006
    Rockville, MD 20847
    www.nichd.nih.gov
    (800) 370-2943
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