Tests for Down syndrome during pregnancy include blood tests and ultrasounds to check for abnormal levels of chemicals and hormones, as well as physical characteristics of the fetus. Chorionic villus sampling and amniocentesis can also be performed. After birth, symptoms of the condition may prompt further testing.
Down syndrome is a genetic disorder that creates an extra copy of chromosome 21 in a person’s cells and is the most common condition causing birth defects. Most tests for Down syndrome are done during pregnancy and use a combination of blood tests and an ultrasound to check levels of chemicals and hormones and to look for any features that may indicate that the fetus has the syndrome of Down. Testing is also done after birth, but usually only if the baby is born with physical characteristics characteristic of a baby with the condition.
These tests can be completed as early as the 11th week of pregnancy, but there is a higher false-positive rate than tests completed during the second or third trimester. During the first trimester, an NHS fetal abnormality screening program is performed using an ultrasound and a blood test. The doctor examines the neck of the fetus and measures the amount of fluid contained within the tissue, which is known as a nuchal translucency screening. A fetus born with Down syndrome usually has a higher amount of fluid than a fetus born without the disease. Blood tests are done with ultrasound to check for abnormal levels of protein-A (PAPP-A) and the hormone chorionic gonadotropin (HCG).
During the second trimester, testing for Down syndrome also requires an ultrasound screening for nuchal translucency and a blood test. The blood test will test not only for PAPP-A and HCG, but will also determine levels of the pregnancy chemicals alpha-fetoprotein, ertriol, and inhibin A. If these levels are abnormal or indicate the fetus may have the genetic disorder, more conclusive tests may be ordered to confirm the results.
Chorionic villus sampling (CVS) is a procedure in which a biopsy of the placenta, which contains the same genetic material as the fetus, is examined for extra chromosomes. The procedure can be performed as early as the 10th week of pregnancy, but carries the risk of miscarriage. CVS can also test for other chromosomal conditions, such as cystic fibrosis and hemophilia.
Aminocentesis is another form of Down syndrome tests that collect amniotic fluid to evaluate fetal chromosomes. A needle is inserted through the abdomen into the amniotic sac to obtain a sample of the fluid for testing. This procedure is usually not done until the 14th or 15th week of pregnancy.
Once the baby has been delivered and has symptoms of the condition, other tests for Down syndrome may be done. Signs of the genetic disorder include muscle weakness, almond-shaped eyes, a flat profile, a single crease in the palm of the hand, and small ears or mouth. The doctor will order a karyotype to determine if there is a chromosomal abnormality in the baby’s cells.
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