Down syndrome is a genetic condition that is caused by an extra full or partial copy of chromosome 21. This condition is the most common genetic condition in the United States and is characterized by lifelong intellectual disabilities, developmental delays, and a distinct physical appearance.
The American College of Obstetricians and Gynecologists recommends offering screening and/or diagnostic tests for Down syndrome to all pregnant women. Though the risk for conceiving a child with Down syndrome increases as you age, you can be screened at any time during your pregnancy, no matter what your age. There are many different types of tests, so you can discuss with your healthcare provider which type of test is the best fit for you.
Screening Tests
Screening tests are used to evaluate the likelihood of a mother carrying a baby with Down syndrome. Many screening tests are offered as a routine part of prenatal care, and they are used to help you make decisions about the course of your pregnancy and whether or not to undergo more specific diagnostic tests.
Screening tests include:
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Blood test: Blood tests are used to measure the levels of pregnancy-associated plasma protein-A (PAPP-A), as well as the pregnancy hormone called human chorionic gonadotropin (HCG). If there are abnormal levels of either of these in your blood, this could indicate that there is something wrong with the pregnancy or the baby.
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Ultrasound: If there are abnormalities with the baby, excess fluid tends to collect in a specific area on the back of the baby’s neck. Nuchal translucency screening tests are used to specifically look in this area if something is suspected to be wrong with the pregnancy.
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Integrated screening test: This is a screening test that is performed in two parts during the first and second trimesters of pregnancy. Together, the results will determine if your baby could have Down syndrome. The first trimester involves a blood test and ultrasound while the second trimester involves measuring your blood levels for four pregnancy-associated substances including alpha fetoprotein, estriol, HCG, and inhibin A.
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Cell-free fetal DNA analysis: This is a test that checks fetal DNA circulating in the mother’s blood. If you are at an increased risk for having a baby with Down syndrome, this test may be recommended to you by your doctor. This procedure is a much more specific way to test for Down syndrome and could help in determining whether more invasive diagnostic tests are necessary.
Diagnostic Tests
If the results of your screening tests are worrisome for you or your doctor, or if you are at an increased risk for having a baby with Down syndrome, then your doctor may recommend undergoing additional testing to confirm the diagnosis. Some diagnostic tests for Down syndrome include:
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Amniocentesis: This involves taking a sample of amniotic fluid that surrounds the fetus using a needle that is inserted through the uterus. The sample is then analyzed for chromosomal abnormalities in the DNA of the fetus. This test can be performed during the second trimester and carries a small risk of miscarriage.
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Chorionic villus sampling (CVS): This test involves taking cells from the placenta to analyze the fetal chromosomes. This test is performed in the first trimester and carries a slightly higher risk for miscarriage than amniocentesis.
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Cordocentesis: This test involves taking fetal blood from a vein in the umbilical cord and examining it for any chromosomal defects. This test can be performed anywhere between 18 and 22 weeks of pregnancy. This test also carries the highest risk of miscarriage when compared to amniocentesis and CVS.
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Chromosomal karyotype: This is a test is a simple blood test that is used to confirm the diagnosis of Down syndrome after the baby is born.
