People with Down syndrome have an extra copy of chromosome 21; Down syndrome is sometimes called trisomy 21. Down syndrome occurs in 9.2 of 10,000 births, and is more likely to occur in the babies of women older than 35. Blood work and ultrasound are used to predict the possibility of the syndrome, which can then be confirmed by more invasive testing.
Triple Screen Testing for Alpha Fetoprotein, Estriol and Human Chorionic Gonadotropin
An abnormal triple screen test, a blood test done between weeks 15 and 18 of pregnancy, diagnoses 60 percent of Down syndrome pregnancies, according to the 2000 report “Down Syndrome: Prenatal Risk Assessment and Diagnosis” published in American Family Physician.
The triple screen is a blood test that scrutinizes three blood tests: alpha fetoprotein, unconjugated estriol and human chorionic gonadotropin (hCG) as well as maternal age to analyze the probability that a fetus has Down syndrome. Alpha fetoprotein and estriol are about 25 percent lower in babies with Down syndrome and hCG is two times higher in the second trimester of pregnancy.
The triple screen is a screening test only. Amniocentesis or chorionic villus sampling, two tests that analyze fetal cells for chromosomal abnormalities, are offered to pregnant women when the triple screen indicates the possibility of Down syndrome.
Amniocentesis is done by withdrawing amniotic fluid from the amniotic sac that surrounds the fetus. Chorionic villus sampling (CVS) removes cells from the placenta for examination.
Nuchal Translucency Scan Test (NT) Screening for Trisomy 21
NT screening is performed between week 11 and 14 of pregnancy. A fold on the fetal neck, called the nuchal fold, is measured via high-resolution ultrasound. The results, along with maternal age, are used to determine the risk of Down syndrome in the fetus.
Around 80 percent of cases of Down syndrome can be predicted by NT screening. Like triple screening blood testing, NT is a screening test only; amniocentesis or CVS should also be done to confirm the diagnosis.
Down Syndrome Second Trimester Level II Fetal Ultrasound to Detect Fetal Anomalies
Second trimester ultrasounds that look more closely at a developing fetus can reveal many potential warning signs of Down syndrome. Many of these signs are not specific to Down syndrome, but a number of warning signs combined with blood tests do give a better indication of the chance of Down syndrome in a fetus.
Congenital heart defects, intrauterine growth retardation, shorter than normal femur and humerus bones, found in the leg and arm, respectively, and a two vessel, rather than three vessel, umbilical cord.
Duodenal atresia, known as a "double bubble" sign, is seen in 20 to 30 percent of Down syndrome babies, according to Medline Plus. In duodenal atresia, the first section of the small bowel doesn’t develop.
Prenatal Testing is a Good Indicator for the Possibility of Down Syndrome in a Fetus
Non invasive testing can diagnose Down syndrome in a majority of fetuses. Amniocentesis or CVS can be used to confirm the diagnosis, but involve more invasive testing.
Sharon Perkins - Sharon is an RN with over 20 years experience in maternal child health, oncology, and ophthalmology. She was the RN coordinator for an ...
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