How Do They Test For Down Syndrome Screening To Diagnosis?

Testing for Down syndrome happens in two main phases: screening and diagnosis. Screening tests, like the first-trimester blood test and nuchal translucency ultrasound, estimate the chance that a baby has Down syndrome. Diagnostic tests, like chorionic villus sampling (CVS) and amniocentesis, provide a definitive answer by analyzing the baby’s chromosomes. Most people start with screening and only move to diagnostic testing if the screening result suggests an increased risk.

What Is the Difference Between Screening and Diagnostic Testing?

Screening tests are risk assessments. They tell you if the chance of Down syndrome is higher or lower than average. They do not tell you if the baby actually has the condition.

Diagnostic tests are definitive. They look directly at the baby’s chromosomes and can confirm or rule out Down syndrome with near-perfect accuracy. Diagnostic tests carry a small risk of miscarriage, which is why they are not offered to everyone.

This distinction matters. A “positive” screening result does not mean the baby has Down syndrome. It means you should consider a diagnostic test to know for sure.

How Do They Test for Down Syndrome During the First Trimester?

First-trimester screening typically combines two parts: a blood test and an ultrasound. This is called the combined screening test.

The blood test measures two proteins: pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG). In pregnancies with Down syndrome, these levels are often unusual. Low PAPP-A and high hCG can suggest an increased risk.

The ultrasound measures the nuchal translucency (NT). This is the clear space at the back of the baby’s neck. A larger-than-typical measurement can indicate a higher risk of Down syndrome and some other chromosomal conditions.

The blood test and ultrasound are combined with your age to produce a single risk estimate. This screening is usually done between 11 and 13 weeks of pregnancy. It detects about 82 to 87 percent of Down syndrome cases.

What Does the Second-Trimester Quad Screen Test Measure?

The quad screen is a blood test done between 15 and 22 weeks of pregnancy. It measures four substances in the mother’s blood:

• Alpha-fetoprotein (AFP)
• Human chorionic gonadotropin (hCG)
• Unconjugated estriol (uE3)
• Inhibin A

In pregnancies with Down syndrome, these levels tend to follow a specific pattern: low AFP, high hCG, low uE3, and high inhibin A.

The quad screen detects about 80 percent of Down syndrome cases. It is often used when someone misses the first-trimester screening window. It can also be combined with first-trimester results for what is called integrated or sequential screening, which slightly improves detection rates.

What Is Cell-Free DNA Screening and How Accurate Is It?

Cell-free DNA (cfDNA) screening, also called NIPT (noninvasive prenatal testing), is a blood test that analyzes small fragments of the baby’s DNA circulating in the mother’s bloodstream. It is typically done after 10 weeks of pregnancy.

This test is highly accurate. For Down syndrome specifically, cfDNA detects about 99 percent of cases. The false positive rate is very low, though it is not zero.

There are important limitations. cfDNA is a screening test, not a diagnostic test. It can return a false positive, especially in early pregnancy or with certain maternal health conditions. It also cannot assess the baby’s overall health or detect structural abnormalities. A positive cfDNA result still requires a diagnostic test for confirmation.

Some research suggests that cfDNA may be less accurate in women with a high body mass index or in pregnancies with twins. Some clinicians recommend it as an option for all pregnant women, while others reserve it for those at higher risk. The evidence supports its use, but it should never be treated as a definitive diagnosis.

What Is Chorionic Villus Sampling (CVS)?

CVS is a diagnostic test performed between 10 and 13 weeks of pregnancy. A doctor removes a small sample of tissue from the placenta. This tissue contains the same chromosomes as the baby.

The sample is collected one of two ways. A thin catheter can be guided through the cervix, or a needle can be inserted through the abdomen. Ultrasound is used to guide the procedure throughout.

The placental tissue is then sent to a laboratory, where the chromosomes are analyzed. Results typically take one to two weeks. CVS provides a definitive diagnosis of Down syndrome.

CVS carries a small risk of miscarriage. The risk is generally considered to be about 1 in 300 to 1 in 500 procedures, though the exact figure varies depending on the study and the experience of the clinician. Because of this risk, CVS is only offered to women whose screening results suggest an increased chance of a chromosomal condition.

What Is Amniocentesis and When Is It Done?

Amniocentesis is another diagnostic test. It is usually performed between 15 and 20 weeks of pregnancy. A doctor inserts a thin needle through the abdomen into the amniotic sac and withdraws a small amount of amniotic fluid.

This fluid contains fetal cells that have been shed from the baby’s skin, urinary tract, and other surfaces. These cells are cultured in a lab and their chromosomes are analyzed.

Amniocentesis is considered the gold standard for prenatal diagnosis of Down syndrome. It is highly accurate, with results that are essentially definitive. Results usually take one to two weeks, though some labs offer a rapid preliminary result within a few days.

The miscarriage risk associated with amniocentesis is slightly lower than CVS, generally estimated at about 1 in 500 to 1 in 900 procedures. This risk has declined over time as ultrasound guidance has improved.

How Are the Results Interpreted?

Screening results are reported as a ratio, such as 1 in 1,000. This means that out of 1,000 pregnancies with the same screening profile, one would be expected to have Down syndrome.

A result of 1 in 300 or higher is typically considered “screen positive” and warrants discussion of diagnostic testing. A result below that threshold is considered “screen negative,” meaning the risk is lower than average but not zero.

Diagnostic results are reported as either normal or abnormal. A normal result shows the expected 46 chromosomes arranged in 23 pairs. Down syndrome occurs when there is an extra copy of chromosome 21, which is called trisomy 21. This is the cause in about 95 percent of Down syndrome cases.

The remaining cases are caused by translocation or mosaicism. Translocation involves a piece of chromosome 21 attaching to another chromosome. Mosaicism means only some cells have the extra chromosome, which can result in milder symptoms.

Which Test Should You Choose?

The choice depends on your priorities and your risk profile. If you want information without any risk to the pregnancy, screening is the appropriate starting point.

If you are at increased risk — for example, if you are 35 or older, or if a screening test came back positive — you may want the certainty that only a diagnostic test can provide.

Some women choose cfDNA as their primary screening test because of its high accuracy. Others prefer the combined first-trimester screening because it also provides useful ultrasound information about the baby’s anatomy.

There is no single right answer. The best choice is the one that aligns with your values, your risk factors, and how much certainty you need. A genetic counselor can help you work through these decisions.

Are There Risks With Any of These Tests?

Blood tests and ultrasounds carry no risk to the pregnancy. They are safe for both mother and baby.

Diagnostic tests, CVS and amniocentesis, carry a small but real risk of miscarriage. This risk is the main reason they are not offered universally.

There is also a small chance that a diagnostic test result will be ambiguous. This can happen with mosaicism or with a condition called confined placental mosaicism, where the placenta has an abnormal chromosome pattern but the baby does not. A genetic counselor can explain these rare outcomes if they occur.