Some parents opt for prenatal screenings or diagnostic tests to understand their baby’s risk of Down syndrome. Here’s what to know about the different types of Down syndrome tests and their effectiveness.
Doctor meeting with pregnant woman
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Down syndrome is a genetic disorder that affects one out of every 700 babies. Most people have 46 chromosomes (clusters of genes) in every cell's nucleus—but those with Down syndrome have a full or partial extra copy of chromosome 21, says Amy Houtrow, M.D., Ph.D., medical director of Children's Hospital of Pittsburgh of UPMC's Rehabilitation Institute and member of the American Academy of Pediatrics' (AAP) Council on Children with Disabilities.

While experts understand the chromosomal abnormalities that cause Down syndrome, they don't know why it happens to certain people. Maternal age may increase your risk: a 25-year-old woman has a one in 1,200 chance of having a baby with Down syndrome, while a 40-year old woman has a one in 100 chance, according to the National Down Syndrome Society. Genetics might also cause one form of the condition called translocation Down syndrome—but this accounts for only 1 percent of all Down syndrome diagnoses. In general, there's not much parents can do to prevent it from happening.

So should you be tested for Down syndrome during pregnancy? Deciding whether to be screened, and which Down syndrome tests to have, is a personal choice. "Some women feel that the more information they have, the better they can prepare themselves, or they may make some tough decisions," says Emily Jean Davidson, M.D., clinical director of the Down Syndrome Program at Boston Children's Hospital. She notes that diagnosing Down syndrome in utero helps doctors tune into whether your baby has any potentially life-threatening related issues, such as congenital heart disease. "But some families decline being screened because they feel the information wouldn't change the course of their pregnancy," she adds.

Regardless of your age and risk factors, the American College of Obstetrics and Gynecology (ACOG) recommends that all pregnant women at least be offered prenatal genetic screening for Down syndrome and other genetic conditions. You can also get diagnostic testing later in the pregnancy. Here's everything you need to know about your options when it comes to the types of Down syndrome tests and their effectiveness.

Prenatal Screening Tests for Down Syndrome

In the first trimester, mothers-to-be can get prenatal screenings to learn their baby's chances of having Down syndrome. These aren't 100 percent accurate; they offer a prediction instead of a guarantee. They can also include false results—some babies with Down syndrome are not identified and others without Down syndrome will be misidentified. However, the screenings have minimal risk to you or your baby, so they might be worth getting.

The first option is a blood test and an ultrasound (called "first trimester combined test"), which is usually done between 10 and 13 weeks of pregnancy. The blood test scans the mother's blood for various substances like hormones and proteins; abnormal levels could indicate a chromosomal abnormality. The ultrasound checks the thickness at the back of the fetus's neck, known as its nuchal translucency. Increased fluid in this region might indicate that your baby has Down syndrome. 

If you're over 35 or have other known risk factors, your doctor may also suggest a fetal DNA test (sold commercially as MaterniT21 or Harmony) during the first trimester. It has a 99 percent accuracy rate because it works by sequencing the small pieces of your baby's DNA that circulates in your bloodstream during pregnancy. 

In your second trimester, another blood test can help detect Down syndrome with moderate accuracy. It's known as the multiple marker screening or quad screening because it checks the level of four different substances in your blood. If a woman gets the first trimester combined test and the quad screening, it's called an "integrated screening test."

Prenatal Diagnostic Tests for Down Syndrome

If your Down syndrome screening tests raise a red flag, you can have a diagnostic test, such as chorionic villus sampling (CVS) or amniocentesis. These tests can accurately tell whether your baby has Down syndrome by detecting chromosomal changes. But while they're almost 100 percent effective, the procedures carry a small risk of miscarriage—up to 1 percent. 

Chorionic Villus Sampling (CVS): This test examines fetal chromosomes from the placenta, and it's usually conducted between 9 and 14 weeks of pregnancy.

Amniocentesis: This test examines amniotic fluid, which surrounds the baby in its amniotic sac. It's typically performed between 15 and 20 weeks of pregnancy.

Cordocentesis: Also called "percutaneous umbilical blood sampling," this test measures blood from the umbilical cord. It gives clearer results than the other tests, but it has a higher risk of miscarriage.

Note that even if prenatal diagnostic testing indicates that your baby has Down syndrome, it can't predict the severity of the condition, according to the CDC. Doctors will determine your child's physical and intellectual symptoms after they're born.

Down Syndrome Test at Birth

Whether you got prenatal Down syndrome tests or not, doctors can diagnose the condition at birth. They might notice certain physical characteristics, such as a flattened face, eyes that slant upward, and a deep crease in the palm. They can also conduct a karyotype (chromosomal analysis that examines the baby's cells through a blood sample) to make a diagnosis, according to the National Down Syndrome Society.