Down syndrome can be diagnosed before or after birth through a combination of screening tests and confirmatory genetic analysis. During pregnancy, screening narrows down who may be at higher risk, while diagnostic procedures like amniocentesis or chorionic villus sampling confirm the diagnosis with near-complete certainty. After birth, doctors often recognize physical characteristics within hours and order a blood test called a karyotype to verify the chromosomal pattern.
Prenatal Screening vs. Diagnostic Testing
The distinction between screening and diagnostic testing is the most important thing to understand about prenatal detection. Screening tests estimate the probability that a baby has Down syndrome. They carry no risk to the pregnancy but can produce false positives and false negatives. Diagnostic tests analyze actual fetal chromosomes and provide a definitive yes or no answer, but they involve a small risk of miscarriage.
The American College of Obstetricians and Gynecologists recommends that all pregnant people be offered both screening and diagnostic testing, regardless of age or risk factors. A positive screening result doesn’t mean the baby has Down syndrome. It means further testing is warranted. And a negative screening result doesn’t guarantee the baby is unaffected, especially if an ultrasound later reveals something unexpected.
First-Trimester Screening
Between weeks 11 and 14 of pregnancy, two types of screening are typically offered together. The first is a blood test that measures specific protein and hormone levels in the mother’s blood. The second is a specialized ultrasound that measures the fluid-filled space at the back of the baby’s neck, called the nuchal translucency. A measurement of 3 millimeters or more (or above the 99th percentile for the baby’s size) signals an increased risk. Combined, these two results generate a risk score.
Cell-free DNA testing, sometimes called NIPT (noninvasive prenatal testing), is the most accurate screening option available. It works by analyzing tiny fragments of the baby’s DNA that naturally circulate in the mother’s bloodstream. A large review of studies published in BMJ Open found that this test correctly identifies Down syndrome 99.3% of the time, with a specificity of 99.9%, meaning false positives are rare. It can be done as early as 10 weeks, though accuracy improves as the pregnancy progresses because the concentration of fetal DNA in the mother’s blood increases over time.
Even with those impressive numbers, cell-free DNA screening is not a diagnosis. It is still a probability estimate, and ACOG is clear that it “is not equivalent to diagnostic testing.” A positive result should always be followed by a diagnostic procedure before making any decisions.
Second-Trimester Screening
If first-trimester screening was missed or if additional information is needed, a second-trimester blood test called the quad screen can be done around weeks 15 to 22. This test measures four substances in the mother’s blood to calculate risk. It detects Down syndrome about 70 to 75% of the time, with a false-positive rate around 5%, making it notably less accurate than cell-free DNA testing. For this reason, many providers now use cell-free DNA as the primary screen when it’s available, though the quad screen remains an option.
A detailed anatomy ultrasound, usually done around week 18 to 20, can also pick up physical markers that raise suspicion. These might include a shorter-than-expected thigh bone, certain heart defects, or excess fluid in the kidneys. None of these findings alone confirm Down syndrome, but they can prompt a recommendation for diagnostic testing.
Diagnostic Procedures During Pregnancy
Two procedures can confirm Down syndrome prenatally by directly examining fetal chromosomes. Chorionic villus sampling (CVS) is available earlier, between weeks 10 and 13. During CVS, a small sample of tissue is taken from the placenta, either through the cervix or through the abdomen with a needle. The miscarriage risk is less than 1 in 100.
Amniocentesis is performed later, typically around week 16. A thin needle passes through the abdomen to collect a small amount of the fluid surrounding the baby. The miscarriage risk is slightly lower than with CVS. Both procedures provide enough genetic material to produce a definitive diagnosis.
The lab can analyze the sample using a karyotype, which creates a visual map of all 46 (or in this case, 47) chromosomes. This method is particularly good at identifying structural rearrangements like translocations and balanced chromosome changes. A newer technique called chromosomal microarray offers higher resolution and can detect very small duplications or deletions that a standard karyotype might miss. However, microarray cannot identify balanced translocations or certain complex chromosome patterns. In practice, labs often use both methods together to get the most complete picture.
Three Genetic Types of Down Syndrome
Diagnostic testing doesn’t just confirm whether Down syndrome is present. It also reveals which of three genetic types is responsible, and this matters for the family.
The vast majority of cases are complete trisomy 21, where every cell in the body contains three copies of chromosome 21 instead of two. This happens because of a random error when the egg or sperm was forming, and it is not inherited.
In less than 5% of cases, the result is mosaic trisomy 21. Here, some cells have three copies of chromosome 21 and others have the typical two. This occurs when a cell division error happens after fertilization, early in embryonic development. People with mosaic Down syndrome may have a wider range of characteristics depending on how many cells are affected.
Translocation trisomy 21 is the type that can run in families. Part of an extra chromosome 21 attaches to another chromosome and gets passed along during cell division. A parent who carries this translocation may have no signs of Down syndrome themselves but can pass the rearranged chromosome to their children. When translocation is identified, doctors typically recommend testing the parents’ chromosomes to determine whether it was a new event or an inherited pattern, which has implications for future pregnancies.
Diagnosis After Birth
Many babies with Down syndrome are identified shortly after delivery based on physical features that are recognizable at birth. The CDC lists several characteristic signs, including a flattened face (particularly the bridge of the nose), almond-shaped eyes that slant upward, a tongue that tends to protrude, a short neck, and small ears, hands, and feet. A single crease running across the palm of the hand, known as a palmar crease, is another common marker, along with poor muscle tone and unusually flexible joints.
No single physical feature is enough for a diagnosis on its own, since some of these traits appear in babies without Down syndrome. When a doctor suspects the condition based on a cluster of these signs, a karyotype test confirms it. For newborns, this involves a simple blood draw from a vein. Lab technicians culture the cells, stain the chromosomes, and arrange them into pairs to check for an extra copy of chromosome 21. This analysis identifies both the presence of trisomy 21 and the specific type (complete, mosaic, or translocation).
In some cases, Down syndrome is not suspected prenatally or at birth and is identified later in infancy when developmental delays or health issues prompt genetic testing. This is relatively uncommon today given the widespread availability of prenatal screening, but it does happen, particularly when families decline or don’t have access to earlier testing.