What are Down syndrome tests?

Down syndrome is a condition in which a person is born with an extra chromosome or an extra piece of a chromosome. The condition causes problems with development, thinking, learning, and physical health. Down syndrome tests can help find this chromosome disorder in unborn babies.

Chromosomes are tiny "packages" in your cells that contain your genes. Genes carry information, called DNA, that controls what you look like and how your body works. Normally, you're born with a set of 46 chromosomes in each cell. The chromosomes are grouped into 23 pairs. Each pair has two chromosomes, one coming from each parent.

Most people with Down syndrome have an extra copy of chromosome 21 in all their cells. This common form of Down syndrome is called trisomy 21. "Trisomy" means having three copies of a chromosome in your cells instead of the usual two. With less common types of Down syndrome, a person may have:

• An extra piece of chromosome 21 attached to another chromosome (translocation Down syndrome)
• An extra chromosome 21 in some, but not all of their cells (mosaic Down syndrome)

The extra chromosome affects the way a baby's brain and body develop. The symptoms may range from mild to severe. Children with Down syndrome usually have some problems with thinking and learning. They may start talking later than other children.

Common physical signs of Down syndrome include a flat face, eyes that slant up, a short neck, small hands and feet, poor muscle tone, and loose joints. Common health problems in childhood include hearing loss, sleep apnea, ear infections, eye diseases, and heart defects.

There are two basic types of tests that help find Down syndrome during pregnancy:

• Prenatal screening tests can show whether your unborn baby has a higher or lower chance of having Down syndrome. If a screening test shows that your baby could have Down syndrome, you'll need another test to find out for sure. There are a few types of screening tests for Down syndrome:
  • First trimester screening includes:
    • A blood test to check your blood for "markers" such as certain proteins that may be linked to a higher risk of Down syndrome
    • An ultrasound image to look for fluid in the back of the baby's neck, which may be a sign of Down syndrome
  • Second trimester screening includes blood tests to check the levels of several markers in your blood that may be a sign of Down syndrome. The test may check three markers (a triple screen test) or four markers (a quadruple screen test).
  • Combined (or integrated) testing uses results from both first and second trimester screening tests to estimate your baby's risk for Down syndrome.
  • Cell-free fetal DNA is a newer test that checks a sample of your blood for unusually large amounts of material from chromosome 21. This test can be done as early as ten weeks. But it's not recommended if you:
    • Have a low risk of having a baby with Down syndrome
    • Are pregnant with two or more babies.
• Prenatal diagnostic tests can diagnose or rule out Down syndrome by checking the chromosomes in a sample of cells. These tests have a small risk of causing a miscarriage, so they're often done after a screening test shows that an unborn baby could have Down syndrome. Diagnostic tests for Down syndrome include:
  • Amniocentesis. This test takes a sample of the amniotic fluid that surrounds the unborn baby in the uterus. The fluid contains cells from the baby that can be tested. The test is usually done between the weeks 15 and 20 of pregnancy.
  • Chorionic villus sampling (CVS). A CVS collects tissue from parts of the placenta called the chorionic villi. The placenta is the organ that grows in the uterus to provide nutrients and oxygen to the baby. Cells from the chorionic villi usually have the same chromosomes as the unborn baby. A CVS may be done between weeks 10 and 13 of pregnancy.
  • Percutaneous umbilical blood sampling (PUBS), also called cordocentesis. This test removes a sample of the baby's blood from the umbilical cord. PUBS gives the most accurate diagnosis of Down syndrome during pregnancy, but it's not commonly used. That's because it has more risks than other tests and it can't be done until later in pregnancy, usually between weeks 18 and 22.

What are the tests used for?

Down syndrome tests are used:

• During pregnancy to:
  • Estimate an unborn baby's risk of Down syndrome.
  • Confirm a diagnosis of Down syndrome in an unborn baby.
• After birth to diagnose or rule out Down syndrome in a newborn that shows signs of the condition. Usually, a karyotype test is done on a sample of the baby's blood. This test checks the number and structure of the baby's chromosomes.
• As part of in vitro fertilization (IVF) to check embryos for chromosome problems before placing them in the uterus to start a pregnancy.

Why do I need a Down syndrome test?

You may need Down syndrome testing during pregnancy if you have a high risk for having a baby with Down syndrome. Your risk may be higher if:

• You were age 35 or older when you got pregnant. The risk of having a baby with Down syndrome increases with age.
• You have a child with Down syndrome or had a diagnosis of Down syndrome in a past pregnancy.
• You have a family history of Down syndrome.
• An ultrasound of your unborn baby showed a possible sign of Down syndrome.

You may choose to be tested even if you don't have a high risk for having a baby with Down syndrome. Your provider or a genetic counselor can help you consider the pros and cons of testing.

What happens during Down syndrome testing?

For a blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

For the first trimester ultrasound, a health care professional will move an ultrasound wand-like device on your belly. Ultrasound uses sound waves to make an image of your unborn baby.

For amniocentesis, a provider inserts a hollow needle through your belly and into your uterus to withdraw a sample of amniotic fluid.

For chorionic villus sampling (CVS), a provider collects a sample of tissue from the placenta with either a needle that's inserted through your belly or a tube that's inserted through your vagina.

For percutaneous umbilical blood sampling (PUBS), a provider inserts a hollow needle through your belly and into the umbilical cord in your uterus. A small blood sample is removed from a vein in the cord.

Will I need to do anything to prepare for the tests?

Blood tests and ultrasounds usually don't require any preparation.

For a CVS or amniocentesis, you may need to drink extra fluid and not urinate before your test so that your bladder is full.

If you're having a PUBS test after 23 weeks of pregnancy, you may need to fast (not eat or drink) for several hours before the test.

Are there any risks to the tests?

Blood tests and ultrasounds have very little risk. After a blood test, you may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

During amniocentesis or CVS, you may feel mild stinging or cramping. Both tests are safe, but they do have a slight risk of causing a miscarriage.

PUBS may cause brief discomfort. This test has a slightly higher risk of causing serious problems than amniocentesis or CVS. Talk with your health care provider about the risks and benefits of testing for Down syndrome.

What do the results mean?

Results from Down syndrome screening tests show your risk of having a baby with Down syndrome:

• A positive screening test result means that your baby's chance of having Down syndrome is higher than average. Your test results may include a number that describes how high the risk is. But a high risk doesn't mean your baby will have Down syndrome. You could still deliver a healthy baby with no chromosomal defects or disorders.
• A negative/normal screening test result means that your baby is unlikely to have Down syndrome. But a screening test can't guarantee that your baby won't have Down syndrome.

Whether or not your screening tests were normal, you may choose to have one or more diagnostic tests. Diagnostic tests can usually tell you whether or not your baby will have Down syndrome. But diagnostic tests cannot predict how much Down syndrome will affect your baby's health and abilities.

You may find it helpful to speak with a genetic counselor before testing and again after you get your results. Genetic counselors are specially trained to help you weigh the risks and benefits of testing. They can also help you understand and what your results mean so you can make informed decisions.

Learn more about laboratory tests, reference ranges, and understanding results.