Down Syndrome Tests

What are Down syndrome tests?

Down syndrome tests check whether a developing fetus may have Down syndrome, a chromosome disorder caused by having an extra copy or piece of chromosome 21.

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 in Down syndrome affects the way a fetus's brain and body develop. This can cause mild or severe problems with development, thinking, learning, and physical health. Children with Down syndrome may also start talking later than other children and have:

• A flat face.
• Eyes that slant up.
• A short neck.
• Small hands and feet.
• Poor muscle tone.
• Loose joints.
• Hearing loss.
• Sleep apnea.
• Ear infections.
• Eye diseases
• Heart defects.

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

• Screening tests, which estimate the chance that a fetus has Down syndrome.
• Diagnostic tests, which confirm whether Down syndrome is actually present by examining the fetus's cells or DNA.

Screening tests are simple and safe, using blood samples and ultrasounds. Diagnostic tests involve collecting cells from the amniotic fluid, umbilical cord, or placenta (the organ that grows in the uterus to provide oxygen and nutrients to the fetus). This makes diagnostic tests more accurate but also slightly riskier.

What are the tests used for?

Down syndrome tests are used for several purposes during and after pregnancy. Each test works in a different way and is done at specific times.

Prenatal screening tests estimate the chance that a fetus has Down syndrome but can't diagnose the condition. There are several types of these screening tests, including:

• First trimester screening (weeks 10-13). This test combines a blood test and an ultrasound.
  • The blood test checks for markers, such as certain proteins, that may be linked to a higher risk of Down syndrome. These can include pregnancy-associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG).
  • The ultrasound measures the thickness of the fluid behind the fetus' neck. Increased fluid thickness can mean a higher chance of Down syndrome. 
• Second trimester screening (weeks 15-20). Often called the quad screen (or quadruple screen test), this screening checks the levels of four substances in your blood. These are: alpha-fetoprotein (AFP), hCG, estriol (a type of estrogen), and inhibin A. Unusual levels of any of these can suggest a higher chance of Down syndrome.
• Combined or integrated screening. This test combines results from both the first and second-trimester screenings. By doing so, this screening test can more clearly estimate your fetus' risk for Down syndrome.
• Cell-free DNA (cfDNA) or noninvasive prenatal screening (NIPS). This test checks small pieces of your fetus' DNA found in your blood for unusually large amounts of material from chromosome 21. It is the most accurate type of test and can be done as early as 10 weeks. Medical experts recommend this test for everyone, regardless of age or risk level.

Prenatal diagnostic tests confirm whether a fetus has Down syndrome. They are usually done after the results of a screening test come back positive and your health care provider needs to confirm a diagnosis. To do this, your provider may order one of the following tests:

• Amniocentesis. This test is usually performed between 15 and 20 weeks of pregnancy. Your provider will collect a small amount of amniotic fluid, which is the fluid that surrounds the fetus. This fluid contains some of your fetus's cells. By collecting a sample of your amniotic fluid, your provider can check the fetal cells for chromosome changes.
• Chorionic Villus Sampling (CVS). This test is usually done between 10 and 13 weeks of pregnancy. Your provider will collect tissue from parts of the placenta called the chorionic villi. Cells from the chorionic villi usually have the same chromosomes as the fetus. So, a provider can tell from the sample if the fetus has any chromosome abnormality.
• Percutaneous Umbilical Blood Sampling (PUBS). This type of test removes a sample of the blood from the umbilical cord of the fetus. It provides very accurate results but carries a higher risk of health complications for the fetus, so it's used only when other tests can't provide clear answers.

Your provider may also test for Down syndrome:

• After birth. Your provider will only test for Down syndrome if your baby shows physical signs of this condition after birth. To do this, they may use a sample of your blood to perform a karyotype test, which can diagnose Down syndrome by checking the number and structure of the baby's chromosomes.
• During in vitro fertilization (IVF). Your provider will test embryos for Down syndrome before they are placed in the uterus.

Why do I need a Down syndrome test?

Medical researchers recommend that everyone, regardless of age and risk, be offered screening for Down syndrome while pregnant. You may talk with your provider about which one of the screening test options works best for you. These options can include first and second trimester screening, cell-free DNA (cfDNA), or noninvasive prenatal screening (NIPS).

You may need additional Down syndrome testing, including diagnostic testing, 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 fetus showed a possible sign of Down syndrome.

You may choose to get a diagnostic test 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 these tests.

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 fetus.

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.

For a newborn karyotype, a provider collects a blood sample from the baby's heel or vein. They will then use that sample to check the number and structure of your baby's chromosomes.

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 (pee) 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.

Your newborn will not need any preparation for a karyotype 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 provider about the risks and benefits of testing for Down syndrome.

There is no risk for a newborn karyotype test.

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 and karyotype 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 what your results mean so you can make informed decisions.

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