Down Syndrome Test: How Doctors Detect Down Syndrome
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Down Syndrome Test

Down Syndrome Test
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Updated on January 24, 2025
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Prenatal testing for Down syndrome can help you understand your baby’s development before birth and provide information to guide your pregnancy journey.

Read on to learn more about prenatal testing for Down syndrome, including how it is performed, how to prepare, and the potential risks.

What Is a Down Syndrome Test?

A Down syndrome test is used to discover whether an unborn baby has Down syndrome, a genetic condition caused by an extra copy of chromosome 21. Down syndrome occurs in about 1 out of every 1,000 live births.

Babies with Down syndrome have a range of physical and intellectual differences. They’re also at higher risk for certain health conditions, such as cancer and heart defects.

Prenatal testing for Down syndrome is entirely optional. The decision to undergo prenatal testing for Down syndrome can be a deeply personal one.

Screening Tests During Pregnancy

Screening tests measure the risk that your baby will have Down syndrome. They don’t result in a diagnosis — they tell you if a condition, like Down syndrome, might be present.

Prenatal screening tests for Down syndrome include:

  • Blood Test This test measures levels of certain hormones and proteins in your blood that may indicate a genetic condition such as Down syndrome. Doctors perform this test during the first or second trimester, along with standard prenatal testing.
  • Ultrasound A nuchal translucency test uses ultrasound to measure fluid on your baby’s neck. More fluid than usual may indicate Down syndrome. Doctors typically perform this test during the first trimester.

Diagnostic Down Syndrome Tests

If your screening tests show a high likelihood of having a baby with Down syndrome, or if you have other risk factors, your doctor may recommend diagnostic testing. The American Academy of Obstetrics and Gynecology recommends that providers offer all pregnant women, regardless of age and risk factors, the option of diagnostic testing first, bypassing screening tests.

Down syndrome diagnostic tests include:

  • Amniocentesis Your provider examines your amniotic fluid for irregularities, specifically three copies of chromosome 21. Amniotic fluid is the liquid that surrounds and protects the fetus in the womb. It contains some cells shed by the fetus so doctors can examine the chromosomal makeup of these cells in order to determine the makeup of the fetus as well. Doctors perform this screening test during the second trimester.

  • Chorionic Villus Sampling (CVS) Your provider takes cells from your placenta to examine your baby’s chromosomes. The placenta is an organ that forms in the womb during pregnancy and contains the same genetic material as the fetus. This test is done during the first trimester.

  • Percutaneous Umbilical Blood Sampling (PUBS), Also Called Cordocentesis Doctors test fetal blood from your baby’s umbilical cord for genetic information. This is the most accurate prenatal test for Down syndrome, and is sometimes used to confirm amniocentesis or CVS results. The test usually occurs between 18 and 22 weeks of pregnancy.

Who Should Get Down Syndrome Tests?

Certain risk factors increase the likelihood of bearing a child with Down syndrome. These include:

  • Advanced maternal age (over 35 years old)
  • Parents who have a chromosomal condition
  • Parents who have another child with Down syndrome or a different chromosomal condition

If you are high risk, you can opt for prenatal testing for Down syndrome.

Factors that may help you decide to have prenatal testing are personal beliefs about raising a child with Down syndrome, the desire for as much information as possible about your baby’s health, and the belief that getting a definitive answer about Down syndrome will ease your anxiety during pregnancy.

How Are Down Syndrome Tests Done?

Blood tests are typically performed in a lab or doctor’s office. A healthcare professional, such as a nurse or phlebotomist, will collect your blood from a vein in your arm or hand.

Doctors perform diagnostic tests, including amniocentesis, CVS, and PUBS, in an outpatient setting — usually the office where you receive your routine prenatal and obstetric care. While the procedure varies for each of these diagnostic tests, your doctor uses ultrasound guidance for each one. Samples are then sent to a lab for testing.

  • Amniocentesis Your doctor inserts a needle through your abdomen and into your uterus to extract amniotic fluid from the amniotic sac.

  • Chorionic Villus Sampling (CVS) Your doctor collects cells from your placenta through your cervix using a thin tube (transcervical) or through your abdomen using a thin needle (transabdominal).

  • Percutaneous Umbilical Blood Sampling (PUBS) Your obstetrician inserts a thin needle through your abdomen and uterus into a blood vessel in your baby’s umbilical cord to collect a small blood sample.

How Do I Prepare for a Down Syndrome Test?

Generally speaking, there aren’t any diet or activity restrictions you should adopt before you undergo screening or diagnostic testing for Down syndrome. Your provider may refer you for genetic counseling with a certified genetic counselor or maternal-fetal medicine specialist before you have a diagnostic test for Down syndrome.

PUBS carries a slight risk of emergency cesarean section (C-section), so your doctor may recommend fasting for several hours before the procedure. For this reason, your obstetrician may perform the procedure in a hospital, but they can also perform it in an outpatient setting.

What Should I Expect During a Down Syndrome Test?

Outpatient diagnostics tests — amniocentesis, CVS, and PUBS — typically take about 30 minutes from start to finish. You can expect to feel some pain when your doctor inserts the needle during these procedures. Mild abdominal cramps are normal during the procedure and may last for a few hours afterward. 

What Are the Potential Risks Associated With a Down Syndrome Test?

Screening tests such as blood tests and ultrasound pose minimal risk.

But diagnostic tests do involve some risk for both mother and baby. These risks include:

  • Infection
  • Bleeding
  • Leaking amniotic fluid
  • Miscarriage, higher risk with PUBS than amniocentesis and CVS
  • Premature labor
  • Limb deformity — CVS performed before 10 weeks of gestation carries a high risk
While miscarriage may be a risk, studies generally show this risk is very low. A fairly recent study found that there was no miscarriage risk associated with diagnostic testing for Down syndrome.


What Type of Care Is Needed After a Down Syndrome Test?

You should take it easy for the rest of the day after a diagnostic test (amniocentesis, CVS, or PUBS) for Down syndrome. Your doctor will let you know when it’s safe to resume your regular activities — typically one to two days after the procedure.

What Are the Possible Complications and Side Effects of Prenatal Testing for Down Syndrome?

Look out for symptoms that may indicate a complication, such as miscarriage or infection.

Let your doctor know right away if you have any of these symptoms:

  • Fever
  • Heavy vaginal bleeding
  • Moderate to severe abdominal cramps that don’t resolve with rest
  • Vaginal discharge or leakage of fluid
  • Swelling or redness at the needle insertion site.

When Can You Expect Down Syndrome Test Results?

After prenatal testing, you’ll receive your results within a few days for screenings or one to two weeks for diagnostic tests like amniocentesis or CVS.

Your doctor will review the results with you, explaining what they mean and any next steps. If the results show Down syndrome or another condition, your healthcare provider may recommend additional tests, like a detailed ultrasound or fetal echocardiogram, to assess your baby’s health.

Doctors typically perform diagnostic tests like CVS or amniocentesis only once per pregnancy unless additional concerns arise. If a Down syndrome diagnosis is confirmed, your doctor may refer you to a genetic counselor or specialist to help you plan for your baby’s care and connect you with resources for ongoing support.

The Takeaway

  • Prenatal testing for Down syndrome is optional, but offers valuable information about your baby.
  • Screening tests estimate the likelihood of Down syndrome, while diagnostic tests provide a definitive answer.
  • If your results show a higher risk of Down syndrome or a confirmed diagnosis, your healthcare provider can guide you through next steps, including additional testing, genetic counseling, and connecting with supportive resources.

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Joy Tanaka, PhD

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Joy Tanaka, PhD, specializes in clinical molecular genetics. She is dedicated to integrating excellent clinical care with cutting-edge medical research for patients with rare and undiagnosed disorders, and is focused on pursuing questions at the interface between genetics and human disease with the goal of developing novel therapies and treatments.

Dr. Tanaka received her PhD from Yale University School of Medicine, where she was a Howard Hughes Medical Institute Medical Research Scholar and recipient of the George Robert Pfeiffer Fellowship for Translational Medicine. She completed her clinical fellowship in cytogenetics and clinical molecular genetics at Columbia University and New York Presbyterian Hospital, and is currently associate clinical laboratory director at Rady Children's Hospital Institute for Genomic Medicine in San Diego, California.

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Resources
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  5. Cordocentesis. Cleveland Clinic. October 31, 2022.
  6. Who is at risk for Down syndrome? Eunice Kennedy Shriver National Institute of Child Health and Human Development. November 30, 2023.
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