Down syndrome, a congenital chromosomal disorder that causes physical anomalies and developmental delays, affects approximately 1 in 700 to 800 people, the Mayo Clinic reports. People with Down syndrome have an extra copy of chromosome number 21. Tests to predict the likelihood that a fetus has Down syndrome are often done early in pregnancy, around 18 weeks. Blood tests and ultrasound give warning signs of Down syndrome, which can be confirmed by chromosomal analysis done by amniocentesis or chorionic villus sampling (CVS). At birth, many physical signs help diagnose Down syndrome.
Maternal Blood Tests
Blood tests done in pregnancy indicate the possibility of Down syndrome, but are not diagnostic of the disorder. Warning signs of Down syndrome are often suggested by an abnormal quadruple screening of four blood tests: alpha fetoprotein (AFP) produced by the fetus; estriol, a form of estrogen formed by the placenta and fetus; hCG (human chorionic gonadotropin), produced by the placenta; and inhibin-A, produced by the ovaries and fetus. According to the March of Dimes, low levels of AFP, high levels of inhibin-A, and abnormal levels of estriol and hCG indicate the possibility of Down syndrome and accurately identify the risk in 80 percent of all cases.
Women who have an abnormal result are given an analysis of their risk of having a fetus with Down syndrome, which takes into account maternal age, among other factors, since the risk of Down syndrome rises with maternal age. Around 5 out of 100 women have an abnormal quad screening, but of those 5, only 4 to 5 percent actually have a fetus with Down syndrome. It's important to remember that blood tests are a screening test only, with many false positives. Further testing to sample fetal chromosomes can diagnose or rule out the disorder.
Prenatal Ultrasound
Several ultrasound measurements indicate the possibility of Down syndrome, but are not diagnostic. Among the earliest ultrasound indicators, sometimes called markers, are inability to see the nasal bone at 11 to 13 weeks, increased nuchal translucency thickness, a measurement of the skin fold at the back of the neck and shorter than normal leg and arm bones. Other indicators include cardiac defects, intestinal abnormalities, cysts in a part of the brain called the choroid plexus and extra fluid in the amniotic sac, known as polyhydramnios, Obstetrics Ultrasound explains.
Newborn Signs
Once a baby is born, it's easier to assess for physical signs of Down syndrome and to do a definitive chromosomal test. Most babies with Down syndrome have a small head, unusually shaped eyes and ears, a large, often protruding tongue, poor muscle tone and small hands with a single crease, known as a simian crease, in the palm, according to the Mayo Clinic.
