Down Syndrome Test

May 14, 2006 Down syndrome is a genetic condition that leads to delays in physical and intellectual development. It is usually caused by an extra chromosome. Normally, children inherit 23 chromosomes from the mother and 23 from the father (for a total of 46 chromosomes). In most cases of Down syndrome, children have an extra copy of the 21st chromosome (i.e., three copies instead of two). Children with Down syndrome have very characteristic traits. Some of the most common signs are: diminished muscle tone, flat face, small depressed nose, upwardly slanted eyes, abnormally shaped ears, excessive flexibility of the joints, enlarged tongue and excessive space between the first (largest) and second toe. About 40 percent of children have some type of heart defect. Children are also at higher risk for infections, vision and hearing problems, respiratory conditions and thyroid problems. According to the National Down Syndrome Society, more than 350,000 Americans have Down syndrome. About one in 733 babies is born with the condition. Risk increases with age. For women under 30, the risk for having a baby with Down syndrome is less than one in 1,000. By 35, the risk increases to one in 400. At 40, the risk jumps to one in 105. And at 49, the risk is one in 12. Prenatal Screening and Diagnosis The most common screening test for Down syndrome is a blood test performed between the 15th and 20th weeks of pregnancy. The test measures the levels of three markers that are secreted by the fetus or placenta: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG) and unconjugated estriol (uE3). The test is not an accurate diagnostic tool. However, low levels of AFP and uE3 and elevated levels of hCG are suggestive of an increased risk for Down syndrome. Three diagnostic tests are used to confirm the presence of Down syndrome or other genetic conditions. Amniocentesis is the removal of a small amount of fetal cells from the amniotic fluid. The test is usually done between the 12th and 20th weeks of pregnancy and carries a slight risk of miscarriage. Chorionic villus sampling (CVS) is the removal of a small amount of chorionic villi (tissue that will help form the placenta). It is done in the 9th to 11th weeks of pregnancy and carries a one to two percent risk of miscarriage. Percutaneous umbilical blood sampling (PUBS) is the removal of a small sample of blood from the fetal umbilical cord. It is the most accurate diagnostic test, but cant be done until much later in the pregnancy - between the 18th to 22nd weeks of pregnancy. The test also carries the greatest risk of miscarriage. The Nuchal Translucency Test Previously, women who wanted to be screened for Down syndrome needed to wait until the second trimester to have the blood test. But now theres another test that can be done in the first trimester - between the 11th and 13th weeks of pregnancy (more specifically between 10 weeks, 6 days and 13 weeks 6 days). Its called nuchal translucency. The nuchal translucency test uses ultrasound pictures to measure the thickness of the fetal neck (an indication of the amount of fluid under the skin). The measurement is compared to standardized results for a fetus of the same age. Elevated levels may indicate the presence of Down syndrome. The test results are combined with those from a blood test that measures the levels of hCG and pregnancy-associated plasma protein A (PAPP-A). The combination gives a risk estimate thats about 90 percent accurate for prediction of Down syndrome. There is a five percent risk of a false positive test result (i.e., the test inaccurately indicates possible Down syndrome), a rate that is similar to the second trimester blood test. Nuchal translucency allows women to be screened for fetal Down syndrome earlier in the pregnancy. Women who test positive can decide if they would like to pursue further screening or diagnostic testing in the second trimester. Elevated nuchal translucency may also be an indication of other problems in the fetus, like heart defects, problems with the lymphatic system, hernia and skeletal deformities. AUDIENCE INQUIRY For information about Down Syndrome, screening and diagnosis: Fetal Medicine Foundation, http://www.fetalmedicine.com/nuchal.htm March of Dimes, http://www.marchofdimes.com, or contact your local chapter National Association for Down Syndrome, http://www.nads.org National Down Syndrome Society, http://www.ndss.org National Institute of Child Health and Human Development, http://www.nichd.nih.gov BIBLIOGRAPHY Avgidou, Kyriaki, M.D., et al., "Prospective First-trimester Screening for Trisomy 21 in 30,564 Pregnancies," American Journal of Obstetrics and Gynecology, June 2005, Vol. 192, No. 6, pp. 1761-1767. Caine, Allan, et al., "Prenatal Detection of Downs Syndrome by Rapid Aneuploidy Testing for Chromosomes 13, 18, and 21 by FISH or PCR Without a Full Karyotype," The Lancet, July 9-15, 2005, Vol. 366, No. 9493, pp. 1267-1278. Chitty, Lyn, et al., "Fetal Nuchal Translucency Scan and Early Prenatal Diagnosis of Chromosomal Abnormalities by Rapid Aneuploidy Screening," British Medical Journal, February 25, 2006, Vol. 332, No. 7539, pp. 452-455. Cleary-Goldman, Jane, M.D., et al., "Screening for Down Syndrome," Obstetrics and Gynecology, January 2006, Vol. 107, No. 1, pp. 11-17. Cuckle, Howard, D.Phil., et al., "Down Syndrome Screening in the First and/or Second Trimester," Seminars in Perinatology, August 2005, Vol. 29, No. 4, pp. 252-257. DAlton, Mary, M.D., and Jane Cleary-Goldman, M.D., "First and Second Trimester Evaluation of Risk for Fetal Aneuploidy," Seminars in Perinatology, August 2005, Vol. 29, No. 4, pp. 240-246. "Down Syndrome Fact Sheet," New York: National Down Syndrome Society, downloaded from website (http://www.ndss.org), April 6, 2006. "Down Syndrome Facts," Wilmette: National Association for Down Syndrome, downloaded from website (http://www.nads.org), April 6, 2006. "Facts About Down Syndrome," Bethesda: National Institute of Child Health and Human Development, downloaded from website (http://www.nichd.nih.gov), April 6, 2006. Fuchs, Karin, M.D., and Jeffrey Peipert, M.D., "First Trimester Down Syndrome Screening," Seminars in Perinatology, August 2005, Vol. 29, No. 4, pp. 195-202. "How is Down Syndrome Diagnosed in a Newborn?" New York: National Down Syndrome Society, downloaded from website (http://www.ndss.org), April 6, 2006. Kagan, Karl, M.D., et al., "Relation Between Increased Fetal Nuchal Translucency Thickness and Chromosomal Defects," Obstetrics and Gynecology, January 2006, Vol. 107, No. 1, pp. 6-10. Macones, George, M.D., and Anthony Odibo, M.D., "First Trimester Screening," Seminars in Perinatology, August 2005, Vol. 29, No. 4, pp. 263-266. "Nuchal Translucency," London: Fetal Medicine Foundation, downloaded from website (http://www.fetalmedicine.com/nuchal.htm), April 6, 2006. "Questions and Answers About Down Syndrome," New York: National Down Syndrome Society, downloaded from website (http://www.ndss.org), April 6, 2006. Reddy, Uma, M.D., and Michael Mennuti, M.D., "Incorporating First-Trimester Down Syndrome Studies into Prenatal Screening," Obstetrics and Gynecology, January 2006, Vol. 107, No. 1, pp. 167-173. Wapner, Ronald, M.D., "First Trimester Screening," Seminars in Perinatology, August 2005, Vol. 29, No. 4, pp. 236-239. Wenstrom, Katharine, M.D., "Evaluation of Down Syndrome Screening Strategies," Seminars in Perinatology, August 2005, Vol. 29, No. 4, pp. 219-224. Wenstrom, Katharine, M.D., "First-Trimester Down Syndrome Screening," Seminars in Perinatology, August 2005, Vol. 29, No. 4, pp. 195-202. Research compiled and edited by Barbara J. Fister