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.

Why this test is done

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.

How to prepare

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.

Sample type

Blood