Down Syndrome Tests
Reviewed
Also called: Down syndrome tests, Trisomy 21 tests, prenatal screening for Down syndrome, diagnostic tests for Down syndrome
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.
Specimen: Specimen type depends on the test: maternal blood, amniotic fluid, chorionic villi, umbilical cord blood, or newborn blood.
Why this test may be ordered
- To estimate the chance that a fetus has Down syndrome (screening tests).
- To confirm whether a fetus or newborn has Down syndrome (diagnostic tests).
- Recommended for all pregnant individuals regardless of age or risk (screening).
- Offered to those with higher risk: age 35 or older, previous child with Down syndrome, family history, or ultrasound findings suggestive of Down syndrome.
Preparation and collection
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.
For a blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. For the first trimester ultrasound, a health care professional will move an ultrasound wand-like device on your belly. For amniocentesis, a provider inserts a hollow needle through your belly and into your uterus to withdraw a sample of amniotic fluid. For a newborn karyotype, a provider collects a blood sample from the baby's heel or vein.
Blood tests and ultrasounds have very little risk. 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.
Understanding results
Results from Down syndrome screening tests show your risk of having a baby with Down syndrome.
- Positive screening test (higher chance): A positive screening test result means that your baby's chance of having Down syndrome is higher than average. Limitation: 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. Next step: Whether or not your screening tests were normal, you may choose to have one or more diagnostic tests.
- Negative/normal screening test (lower chance): A negative/normal screening test result means that your baby is unlikely to have Down syndrome. Limitation: But a screening test can't guarantee that your baby won't have Down syndrome. Next step: Whether or not your screening tests were normal, you may choose to have one or more diagnostic tests.
- Diagnostic or karyotype test result: Diagnostic and karyotype tests can usually tell you whether or not your baby will have Down syndrome. Limitation: But diagnostic tests cannot predict how much Down syndrome will affect your baby's health and abilities. Next step: They can also help you understand what your results mean so you can make informed decisions.
- Screening tests only estimate the chance of Down syndrome; they cannot diagnose the condition.
- A negative screening result does not guarantee that the baby will not have Down syndrome.
- Diagnostic tests confirm whether Down syndrome is present but cannot predict the severity of its effects on health and abilities.
- Diagnostic tests (amniocentesis, CVS, PUBS) carry a small risk of miscarriage or other complications.
What to do next
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.
- What do my screening test results mean for my baby's chance of having Down syndrome?
- Should I consider a diagnostic test after a positive screening result?
- What are the risks and benefits of diagnostic testing?
- How will a diagnosis of Down syndrome affect my baby's health and development?
- Can a genetic counselor help me understand my options?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.