How To Test For Down Syndrome During Pregnancy?

Prenatal testing for Down syndrome has changed dramatically in the last decade. Most pregnant people in the United States now receive screening tests first, not diagnostic ones. Screening tells you the chance that your baby has Down syndrome. Diagnostic tests tell you for sure. The choice of which test to take — and whether to take any at all — is personal and depends on your medical history, age, and values.

What Is the Difference Between Screening and Diagnostic Tests?

This is the most important distinction to understand. Screening tests estimate risk. They do not give a yes or no answer. Diagnostic tests confirm whether Down syndrome is actually present.

Screening tests are safe for the baby. They involve a blood draw from the pregnant person or an ultrasound. There is no risk of miscarriage. Diagnostic tests require a needle to collect a sample of amniotic fluid or placental tissue. The CDC reports that the miscarriage risk from an amniocentesis is about 1 in 500 to 1 in 1,000 procedures.

Most doctors start with screening. If the screening result suggests a higher chance, they offer a diagnostic test. Some people skip screening entirely and go straight to diagnosis, especially if they are over 35 or have a family history of chromosomal conditions.

How To Test For Down Syndrome During Pregnancy With Screening

The most common screening approach in the United States is called combined first-trimester screening. It happens between weeks 11 and 13 of pregnancy. It has two parts: a blood test that measures two proteins from the placenta, and an ultrasound that measures the fluid behind the baby’s neck, called nuchal translucency.

Research published in the New England Journal of Medicine found that this combined screening detects about 85% of Down syndrome cases. It also has a false positive rate of about 5%. That means 5 out of 100 women will get a “high risk” result even though their baby does not have Down syndrome.

Another screening option is cell-free DNA testing, often called NIPT. This is a blood test that analyzes small fragments of the baby’s DNA circulating in the pregnant person’s blood. It can be done as early as week 10. Studies in the journal Ultrasound in Obstetrics & Gynecology show NIPT detects more than 99% of Down syndrome cases with a false positive rate below 0.5%. It is more accurate than the first-trimester combined screen, but it is also more expensive and not always covered by insurance.

What Do Diagnostic Tests Involve?

Two diagnostic tests are available. Both are invasive procedures that require a doctor to guide a thin needle using ultrasound images.

Chorionic villus sampling (CVS) is done between weeks 10 and 13. The doctor collects a small piece of placental tissue. Results take about one to two weeks. CVS can detect Down syndrome and other chromosomal conditions with near 100% accuracy. The American College of Obstetricians and Gynecologists states the miscarriage risk is about 1 in 455 procedures.

Amniocentesis is done between weeks 15 and 20. The doctor collects a small amount of amniotic fluid that contains fetal cells. Results take about two weeks. Amniocentesis also has near 100% accuracy for Down syndrome. The miscarriage risk is slightly lower than CVS, at roughly 1 in 500 to 1 in 1,000.

Both tests can cause cramping or spotting afterward. Serious complications are rare but include infection or leaking of amniotic fluid. Your doctor should explain these risks before you decide.

Who Should Consider Diagnostic Testing?

Diagnostic testing is not recommended for everyone. The American College of Obstetricians and Gynecologists recommends offering diagnostic testing to all pregnant people, regardless of age. But in practice, most doctors reserve it for people with higher risk factors.

Common reasons to consider diagnostic testing include a positive screening result, being 35 or older at delivery, having a previous child with Down syndrome, or having a known chromosomal rearrangement in either parent. Some people also choose diagnostic testing for peace of mind even without risk factors.

It is important to know that diagnostic testing is optional. You can decline it even if your screening result is positive. Some people prefer to wait until birth for a definitive answer. Others want certainty during pregnancy to prepare or make decisions. There is no wrong choice here.

What About the Quad Screen and Other Older Tests?

The quad screen is a blood test done between weeks 15 and 22. It measures four substances in the pregnant person’s blood. It detects about 75% to 80% of Down syndrome cases. This is lower than the first-trimester combined screen and much lower than NIPT.

Some clinics still offer the quad screen, especially if someone misses the 11-to-13-week window for first-trimester screening. But its use has declined significantly since NIPT became available. The CDC notes that the quad screen has a false positive rate of about 5% to 8%, meaning many women get unnecessary worry from it.

Another older approach is the triple screen, which measures only three substances. It is rarely used now. If your doctor offers these older tests, ask why NIPT or the first-trimester combined screen is not available instead.

Common Misconceptions About Down Syndrome Testing

A widespread myth is that screening tests are always wrong or always right. Neither is true. A positive screening result does not mean your baby definitely has Down syndrome. It means the chance is higher than average. Most women with a positive screening result go on to have a baby without Down syndrome.

Another misconception is that NIPT is diagnostic. It is not. NIPT is a screening test, even though it is highly accurate. A positive NIPT result should always be confirmed with CVS or amniocentesis before making any decisions. Some women have terminated pregnancies based on a positive NIPT alone, only to later learn the result was false. This is tragic and avoidable.

Some people also believe that testing for Down syndrome is mandatory. It is not. Every pregnant person has the right to decline any or all testing. Some choose not to test because they would not change their pregnancy management regardless of the result. Others decline due to personal or religious beliefs. Your doctor should respect your choice.

What Happens After a Positive Diagnosis?

If a diagnostic test confirms Down syndrome, you will have decisions to make. Some parents choose to continue the pregnancy and prepare for a child with special needs. Others choose to terminate. Both are valid options, and the decision is deeply personal.

If you continue the pregnancy, your doctor will likely refer you to a maternal-fetal medicine specialist. You may also meet with a genetic counselor, a pediatric cardiologist, and a neonatologist. About half of babies with Down syndrome are born with a heart defect, so a fetal echocardiogram is often recommended around week 18 to 22.

Organizations like the National Down Syndrome Society and the Down Syndrome Diagnosis Network offer support and resources. Many parents find it helpful to connect with other families who have children with Down syndrome. Your hospital or genetic counselor can provide local and national contact information.