Down syndrome can be diagnosed before or after birth through a combination of screening tests, diagnostic procedures, and genetic analysis. During pregnancy, screening narrows down who has a higher chance of carrying a baby with the condition, while diagnostic tests like amniocentesis provide a definitive answer. After birth, doctors often recognize physical signs within the first hours of life and confirm with a blood test.
Prenatal Screening vs. Diagnostic Tests
There’s an important distinction between screening and diagnosis during pregnancy. Screening tests estimate your chance of having a baby with Down syndrome. They carry no risk to the pregnancy but can’t give a yes-or-no answer. Diagnostic tests analyze the baby’s actual chromosomes and provide a definitive result, but they involve a needle entering the uterus, which carries a small risk of miscarriage.
The American College of Obstetricians and Gynecologists recommends that all pregnant people be offered screening for common chromosome conditions, regardless of age. This is a shift from older guidelines that reserved certain tests for those over 35.
First Trimester Screening (Weeks 11 to 13)
The earliest routine screening happens between weeks 11 and 13 of pregnancy. It combines a blood draw with a specialized ultrasound. The ultrasound measures a small pocket of fluid at the back of the baby’s neck, called the nuchal translucency. In a typical pregnancy, this measurement ranges from about 1.2 mm at week 11 to 1.5 mm at nearly 14 weeks. In pregnancies affected by Down syndrome, this fluid space tends to be significantly larger, averaging around 2.67 times the expected value.
The blood test measures two substances. The first is a protein made by the placenta that tends to be much lower than normal in Down syndrome pregnancies, dropping to roughly 15% of the typical level. The second is a hormone fragment that tends to run about twice as high as expected. A computer model combines these three markers with your age to generate a risk estimate, such as 1 in 500 or 1 in 50. This combined first trimester screen detects about 90% of Down syndrome pregnancies.
Second Trimester Screening (Weeks 15 to 22)
If you missed the first trimester window or your provider uses a different protocol, a second trimester blood test called the quad screen is available. It measures four substances in your blood: alpha-fetoprotein, a specific estrogen, a hormone, and a protein called inhibin-A. Each has a characteristic pattern when Down syndrome is present. The results, combined with your age, generate a risk score. A commonly used cutoff is 1 in 250: scores higher than that are flagged as increased risk.
Some providers use an integrated approach, combining results from both first and second trimester blood work to improve accuracy.
Cell-Free DNA Testing (NIPT)
Non-invasive prenatal testing, often called NIPT, is the most accurate screening option available. It works by analyzing tiny fragments of the baby’s DNA that circulate in your bloodstream, typically starting around week 10. For Down syndrome, NIPT has a pooled sensitivity of 99.3% and a specificity of 99.9%, meaning it catches nearly every affected pregnancy and rarely flags unaffected ones.
That said, NIPT is still a screening test, not a diagnostic one. In the general pregnant population, where Down syndrome is relatively uncommon, as many as 20% of positive NIPT results turn out to be false positives. That’s not a flaw in the test itself but a mathematical reality: when a condition is rare, even a highly accurate test will produce some incorrect positive results. A positive NIPT always needs confirmation through a diagnostic procedure before any decisions are made.
Diagnostic Testing: Amniocentesis and CVS
Two procedures can definitively confirm or rule out Down syndrome during pregnancy. Chorionic villus sampling (CVS) is typically performed at 10 to 12 weeks of pregnancy. A small sample of tissue is taken from the placenta, which shares the baby’s genetic makeup. Amniocentesis is usually done at 15 to 18 weeks and involves withdrawing a small amount of amniotic fluid, which contains fetal cells. Both procedures carry a small risk of miscarriage, generally estimated at less than 1%.
The collected cells are then sent for genetic analysis. The standard approach is a karyotype, which produces a photograph of all 46 (or in this case, 47) chromosomes arranged in pairs. This is considered the gold standard but requires cells to be grown in a lab, so results take 10 to 14 days. For faster results, a technique called FISH can identify the extra chromosome 21 within one to three days, though it doesn’t provide the complete chromosome picture a full karyotype does.
In some cases, a more detailed analysis called chromosomal microarray may be used. This technology detects smaller genetic changes that a standard karyotype might miss, including low-level mosaicism, where only some cells carry the extra chromosome.
How Down Syndrome Is Identified at Birth
Many babies with Down syndrome are recognized at delivery based on a cluster of physical features. No single feature is diagnostic on its own, since each can appear in babies without the condition, but the combination is distinctive. Common signs include a flattened facial profile (particularly the bridge of the nose), almond-shaped eyes that slant upward, a short neck, small ears, hands, and feet, and a single crease running across the palm. Babies with Down syndrome also tend to have noticeably low muscle tone, feeling “floppy” when held.
If a doctor suspects Down syndrome based on these features, a blood sample is drawn for a karyotype. This confirms the diagnosis genetically and also reveals which type of Down syndrome is present, which matters for the family’s future reproductive planning.
Three Genetic Types of Down Syndrome
The karyotype doesn’t just confirm the diagnosis. It identifies which of three genetic forms is responsible.
- Standard trisomy 21 accounts for about 95% of cases. Every cell in the body carries three copies of chromosome 21 instead of two, caused by an error during cell division before or right at conception.
- Translocation accounts for roughly 5% of cases. The extra chromosome 21 material is attached to another chromosome, usually chromosome 14. This is the only form that can sometimes be inherited from a parent who carries the rearrangement without being affected.
- Mosaic trisomy 21 occurs in about 2% of individuals. Only some cells have the extra chromosome, while others have the typical 46. People with mosaic Down syndrome may have milder features, though there’s wide variation.
Knowing the type matters most for translocation Down syndrome. If the karyotype shows a translocation, both parents are usually offered genetic testing to determine whether one of them carries a balanced version of the rearrangement, which would increase the chance of recurrence in future pregnancies. In standard trisomy 21 and mosaic forms, the recurrence risk is generally low.