Testing for Down syndrome happens in two stages: screening tests that estimate your baby’s risk, and diagnostic tests that give a definitive yes or no answer. Screening is offered to all pregnant people regardless of age, typically starting as early as 10 weeks. If a screening result comes back high-risk, a diagnostic test can confirm whether the baby actually has the condition.
Understanding the difference between these two categories is the single most important thing when navigating prenatal testing. A screening test tells you the likelihood. A diagnostic test tells you for certain. No screening test, no matter how advanced, can replace a diagnostic one.
Screening vs. Diagnostic Tests
Screening tests are blood draws and ultrasounds. They carry no risk to the pregnancy and can flag whether your baby has a higher chance of having Down syndrome. But they can also produce false positives (flagging a risk that isn’t there) and, less commonly, false negatives (missing a case). Think of screening as a filter that helps you decide whether to pursue more definitive testing.
Diagnostic tests analyze actual chromosomes from fetal cells. They can confirm with near-complete certainty whether your baby has an extra copy of chromosome 21. The tradeoff is that they require collecting a small sample of tissue or fluid from inside the uterus, which carries a small risk of miscarriage.
First Trimester Screening (Weeks 11 to 13)
The earliest standard screening combines a blood test with a specialized ultrasound called a nuchal translucency (NT) scan. The ultrasound measures a pocket of fluid at the back of the baby’s neck. All babies have some fluid there, but a measurement above 3 millimeters at 12 weeks is considered elevated and prompts a conversation about further testing. The blood test measures two pregnancy hormones, and those levels are combined with the ultrasound measurement and your age to generate a risk score.
This combined screening catches roughly 80 to 85 percent of Down syndrome cases. It’s widely available and noninvasive, making it a common starting point.
Cell-Free DNA Screening (NIPT)
Non-invasive prenatal testing, or NIPT, is a blood test available from 10 weeks onward. It works by analyzing tiny fragments of your baby’s DNA that circulate in your bloodstream. A lab counts those fragments to detect whether there’s an excess of chromosome 21 material.
NIPT is the most accurate screening option available. A large meta-analysis published in BMJ Open found it detects 99.3% of Down syndrome cases, with a specificity of 99.9%, meaning false positives are rare. That combination of high sensitivity and high specificity is why NIPT has become the go-to screening test for many pregnancies.
But here’s the critical nuance: a “high-risk” NIPT result does not mean your baby definitely has Down syndrome. The chance that a positive result is truly positive (called positive predictive value) depends heavily on your age. For a 25-year-old tested at 12 weeks, a high-risk NIPT result is correct only about 28% of the time. For a 40-year-old at the same gestational age, that number jumps to roughly 85%. This is because Down syndrome is more common in older pregnancies, so the test is working with a higher baseline probability. This is why a diagnostic test is always recommended to confirm a positive NIPT.
NIPT also has a known blind spot. The DNA fragments it reads come from the placenta, not directly from the baby. In 1 to 2 percent of pregnancies, the placenta and baby have different genetic makeups, a situation called confined placental mosaicism. This can lead to both false positives and false negatives, particularly in cases of mosaic Down syndrome, where only some of the baby’s cells carry the extra chromosome.
Second Trimester Screening (Weeks 15 to 22)
If you missed first trimester screening or want additional information, the quad screen is a blood test typically done between weeks 15 and 18. It measures four substances in your blood: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), a form of estrogen called estriol, and inhibin-A. Abnormal patterns in these four markers can indicate a higher risk of Down syndrome or other conditions like neural tube defects.
The quad screen is less accurate than NIPT, catching around 80% of cases with a higher false positive rate. It’s still widely used, especially when NIPT isn’t covered by insurance or isn’t available. A detailed anatomy ultrasound around 18 to 20 weeks can also identify physical markers sometimes associated with Down syndrome, though ultrasound alone isn’t a reliable screening tool.
Diagnostic Tests: CVS and Amniocentesis
Only two prenatal tests can definitively diagnose Down syndrome. Both involve collecting fetal cells and analyzing their chromosomes directly.
Chorionic villus sampling (CVS) is performed between 10 and 12 weeks. A small sample of tissue is taken from the placenta, either through the cervix or through the abdomen with a needle. Results typically come back within one to two weeks.
Amniocentesis is performed between 15 and 18 weeks. A thin needle guided by ultrasound draws a small amount of amniotic fluid, which contains fetal cells. The chromosomes in those cells are then mapped in a test called a karyotype, which shows whether there’s an extra chromosome 21.
The primary concern with both procedures is miscarriage. For amniocentesis performed by an experienced provider with ultrasound guidance, the risk is about 0.1% to 0.3%. Risk is higher when amniocentesis is performed before 15 weeks, which is why CVS is the preferred option for early diagnostic testing. Both procedures carry similar overall risk profiles when done within their recommended windows.
Insurance Coverage for NIPT
Most major insurers now cover NIPT for singleton pregnancies regardless of age or risk level. UnitedHealthcare, Anthem, Aetna, Cigna, and Blue Cross Blue Shield’s federal plan all cover it for any single pregnancy. Some, like Aetna and Anthem, also cover it for twin pregnancies. However, a few plans, including TRICARE and Molina Healthcare, still restrict coverage to high-risk pregnancies only, which typically means you’re 35 or older, have a history of a prior pregnancy with a chromosomal condition, or had an abnormal result on another screening test.
If your insurance limits coverage, the out-of-pocket cost for NIPT can vary significantly depending on the lab. It’s worth calling both your insurer and the testing lab directly to get a clear price before proceeding.
Diagnosis After Birth
Some families don’t do prenatal testing, or a case is missed by screening. Down syndrome can also be diagnosed after delivery. A physical exam in the first 24 hours is usually enough for a doctor to suspect the condition based on characteristic features like low muscle tone, a flat facial profile, and an upward slant to the eyes.
To confirm, a blood sample is taken from the baby for a chromosomal karyotype. This maps all 46 (or in this case, 47) chromosomes and identifies whether there’s a full extra copy of chromosome 21, a partial copy (translocation), or a mix of normal and trisomy cells (mosaicism). The type matters because it affects recurrence risk in future pregnancies and can influence the range and severity of associated health concerns.
Choosing a Testing Path
There’s no single correct sequence. Some people start with NIPT because of its accuracy and early availability. Others begin with first trimester combined screening because it’s more widely covered or because they want the ultrasound component. Some skip screening entirely and go straight to CVS or amniocentesis if they know they want a definitive answer, particularly if they’re over 35 or have a family history.
What matters is understanding what each test can and can’t tell you. A low-risk screening result is reassuring but not a guarantee. A high-risk result, especially from NIPT, is a reason to pursue diagnostic testing rather than a reason to panic. Roughly 7 out of 10 positive NIPT results in younger women turn out to be false alarms once confirmed by amniocentesis or CVS.