Prenatal Screening Tests for Down Syndrome
Prenatal screening tests give parents information about their risk of having a child with Down syndrome.
Expanded alpha-fetoprotein (E-AFP) is a simple blood test done between the 15th and 20th week of pregnancy. It screens for several chromosomal abnormalities. This test is often combined with an ultrasound to identify possible physical markers associated with Down syndrome.
Nuchal translucency (NT) screening test can be done between the 11th and 14th weeks, earlier than the E-AFP. It is a high-resolution ultrasound exam of the nuchal translucency. This is the fluid found under the skin at the back of the fetus’ neck. More fluid tends to accumulate here in babies with abnormalities. The result of the NT scan and a blood test is combined with the pregnant woman’s age to determine any increased risk for Down syndrome and other chromosomal abnormalities.
If a screening test is positive, the following diagnostic tests will give precise information about the chromosomal makeup of the fetus or infant.
Amniocentesis involves inserting a needle with ultrasound guidance through the abdomen to withdraw amniotic fluid. This fluid is then checked for chromosomal variations. This test is done between the 16th and 20th weeks of pregnancy.
Chorionic villus sampling (CVS) involves removing a small piece of tissue from the placenta through a needle or catheter with ultrasound guidance. CVS can be done earlier than the amniocentesis at about the 10th to 13th week of pregnancy.