What prenatal screening tests can be done to detect Down Syndrome?

Question: What prenatal screening tests can be done to detect Down Syndrome?

Answer: Prenatal screening for Down syndrome is available. There is a relatively simple, noninvasive screening test that examines a drop of the mother's blood to determine if there is an increased likelihood for Down syndrome. This blood test measures the levels of three markers for Down syndrome: serum alpha feto-protein (MSAFP), chorionic gonadotropin (hCG), and unconjugated estriol (uE3). While these measurements are not a definitive test for Down syndrome, a lower MSAFP value, a lower uE3 level, and an elevated hCG level, on average, suggests an increased likelihood of a Down syndrome fetus, and additional diagnostic testing may be desired.

Diagnostic Testing For Down Syndrome

There are several prenatal diagnostic tests that can be performed to determine the occurrence of Down syndrome. These tests include amniocentesis, chorionic villus sampling (CVS), and percutaneous umbilical blood sampling (PUBS). However, before undergoing any of these diagnostic tests, patients and their families should seek detailed genetic counseling to discuss their family history in relationship to the risks and benefits of performing these diagnostic procedures.

Amniocentesis, the removal and analysis of a small sample of fetal cells from the amniotic fluid, is widely available and involves a lower risk of miscarriage than chorionic villus sampling. However, amniocentesis cannot be done until the 14th to 18th week of pregnancy, and it usually takes additional time to determine whether the cells contain extra material from chromosome 21.

Chorionic villus sampling, conducted at 9 to 11 weeks of pregnancy, involves extracting a tiny amount of chorionic villi, tissue extensions that will eventually develop into a placenta. The tissue can be tested for the presence of extra material from chromosome 21. The villi can be obtained through the pregnant woman's abdomen or cervix. This type of sampling carries a 1-2% risk of miscarriage.

The third diagnostic method, percutaneous umbilical blood sampling or PUBS, is the most accurate method and can be used to confirm the results of CVS or amniocentesis. However, PUBS cannot be performed until later in the pregnancy, during the 18th to 22nd weeks, and carries the greatest risk of miscarriage.

New prenatal diagnostic techniques are currently being developed. The NICHD has supported the development of a new, noninvasive test performed during the first trimester of pregnancy, that samples and separates fetal cells from the mother's blood. The goal is to compare the accuracy of this type of cellular level analysis with results obtained by amniocentesis or CVS.

Diagnostic Tests For Down Syndrome

Amniocentesis

• The removal and analysis of a small sample of fetal cells from the amniotic fluid.
• Cannot be done until the 14-18th week of pregnancy
• Lower risk of miscarriage than chorionic villus sampling

Chorionic Villus Sampling (CVS)

• Extraction of a tiny amount of fetal tissue at 9 to 11 weeks of pregnancy
• The tissue is tested for the presence of extra material from chromosome 21
• Carries a 1-2% risk of miscarriage

Percutaneous Umbilical Blood Sampling (PUBS)

• Most accurate method used to confirm the results of CVS or amniocentesis.
• The tissue is tested for the presence of extra material from chromosome 21
• PUBS cannot be done until the 18-22nd week
• Carries the greatest risk of miscarriage

Researchers outside the NICHD are also developing a new method of diagnosis, called preimplantation diagnosis or blastomere analysis before implantation (BABI), which allows clinicians to detect chromosome imbalances before an embryo is implanted during in vitro fertilization. This technique would primarily be used in couples who are at risk of passing on X-linked disorders, couples who have suffered repeated terminations of pregnancy, subfertile couples, or those at risk for single gene disorders. This technique, which allows the clinician to provide a genetic diagnosis prior to implantation, has been successful so far for cystic fibrosis, Tay Sachs disease, and Lesch-Nyhan syndrome. BABI allows a couple to begin their pregnancy knowing that the fetus is unaffected with the genetic disease of concern. For couples at high risk, this procedure provides an alternative to prenatal testing in the first or second trimester.

^{reproduced from the NIH National Institute of Child Health & Human Development}

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