Prenatal genetic testing is highly accurate, but how accurate depends on which test you’re talking about. Screening tests like NIPT (noninvasive prenatal testing) detect over 96% of the most common chromosomal conditions, while diagnostic tests like amniocentesis approach near-perfect accuracy. The distinction between these two categories, screening versus diagnostic, is the single most important thing to understand about prenatal genetic testing.
Screening Tests vs. Diagnostic Tests
Prenatal genetic tests fall into two groups. Screening tests estimate the probability that your baby has a chromosomal condition. They analyze fragments of placental DNA circulating in your blood and flag pregnancies that may need further investigation. A positive screening result does not mean your baby has a condition. It means the risk is high enough to warrant a diagnostic test.
Diagnostic tests, on the other hand, directly sample fetal cells. Amniocentesis draws a small amount of amniotic fluid, and chorionic villus sampling (CVS) takes a tiny piece of placental tissue. Because these tests examine actual fetal chromosomes rather than estimating risk from blood markers, they provide definitive answers. When a diagnostic test returns a result, that result is almost always correct.
How Accurate NIPT Really Is
NIPT is the most accurate screening test available during pregnancy, and it has largely replaced older blood screening methods like the quad screen, which had significantly higher false positive rates. In a study of nearly 69,000 pregnancies, NIPT detected 98.2% of Down syndrome (trisomy 21) cases with a specificity of 99.9%. For trisomy 18 (Edwards syndrome), detection was 96.2% with 99.9% specificity. For trisomy 13 (Patau syndrome), sensitivity was 100% with 99.95% specificity.
Those numbers sound nearly perfect, and for ruling conditions out, they essentially are. If your NIPT comes back low-risk, you can be very confident. But the flip side is more nuanced. Because these conditions are rare in the general population, even a small false positive rate means that a meaningful number of “high-risk” NIPT results turn out to be wrong. This is why a positive NIPT result always needs confirmation through amniocentesis or CVS before any decisions are made.
Where NIPT Is Less Reliable
NIPT performs best for the common trisomies. When labs offer expanded panels that screen for rarer conditions like microdeletion syndromes, accuracy drops considerably. For 22q11.2 deletion syndrome (DiGeorge syndrome), one of the more commonly screened microdeletions, the positive predictive value has ranged from as low as 18% to as high as 94% depending on the lab’s methods. That means in some settings, the majority of positive results for this condition are false alarms. If your NIPT flags a microdeletion, the result is far less certain than it would be for Down syndrome.
How Accurate Amniocentesis and CVS Are
Amniocentesis performed in the second trimester (typically around 15 to 20 weeks) is the gold standard. Large reviews have found essentially zero false negatives for standard chromosomal conditions when performed at this stage. In rare instances, very subtle abnormalities at the tips of chromosomes or unusual sex chromosome patterns have been missed, but these cases are exceptional. For the conditions most parents are concerned about, second-trimester amniocentesis is as close to 100% accurate as any medical test gets.
CVS, performed earlier (usually 10 to 13 weeks), is also highly accurate but has a slightly higher rate of ambiguous results. Lab failure occurs in about 1.7% of CVS cases compared to just 0.07% after amniocentesis. CVS also has a higher chance of picking up chromosomal abnormalities that exist only in the placenta, not the baby, which can lead to false positives that require follow-up testing.
The tradeoff for this accuracy is a small procedural risk. Amniocentesis carries a miscarriage risk of roughly 0.1% to 0.3% when performed by an experienced provider using ultrasound guidance. CVS carries a similar or slightly higher risk depending on the approach used.
Why NIPT Results Sometimes Get It Wrong
NIPT doesn’t actually test fetal DNA directly. It analyzes cell-free DNA in your blood that comes from the placenta. Most of the time, the placenta’s genetic makeup matches the baby’s. But in about 0.6% to 1% of pregnancies, the placenta contains cells with a different chromosomal makeup than the baby. This is called confined placental mosaicism. When it happens, NIPT reads the placental abnormality and flags a problem that doesn’t actually exist in the fetus. One study that investigated false positive NIPT results found that confined placental mosaicism explained 6 out of 10 cases.
Occasionally, the source of a false result is the mother herself. Some women carry low-level chromosomal mosaicism in their own cells, or rarely, a previously undetected tumor can shed abnormal DNA into the bloodstream. These are uncommon but real sources of error that have nothing to do with the baby.
Factors That Affect NIPT Accuracy
The reliability of NIPT depends heavily on fetal fraction: the percentage of cell-free DNA in your blood that comes from the placenta rather than from your own cells. At 10 weeks of pregnancy, fetal fraction averages about 10%, which is generally enough for reliable results. Labs set a minimum threshold, and if your sample falls below it, you’ll receive a “no-call” result rather than an inaccurate one. This happens in about 2% to 5% of tests.
Two factors most strongly influence fetal fraction. The first is gestational age. Fetal fraction rises throughout pregnancy, increasing about 0.44% per week between weeks 10 and 12.5, then more slowly until around week 20, after which it climbs more steeply. Testing too early can mean insufficient fetal DNA for a reliable read. The second factor is body weight. Higher maternal BMI consistently correlates with lower fetal fraction, likely because women with more body fat produce more of their own cell-free DNA, diluting the placental signal. If your first NIPT attempt returns a no-call result, your provider may suggest waiting a few weeks and retesting, since the fetal fraction naturally increases with time.
Accuracy in Twin Pregnancies
NIPT works in twin pregnancies, though with some added complexity. For Down syndrome screening in twins, sensitivity is about 99%, comparable to singleton pregnancies. However, twins have a higher rate of no-call results due to lower individual fetal fractions.
Identical twins share the same genetic makeup, so NIPT performs as well or better than it does for singletons. Fraternal twins are trickier because each fetus contributes its own portion of cell-free DNA, and those contributions aren’t necessarily equal. One twin might have an adequate fetal fraction while the other falls below the threshold for reliable testing. Newer methods that use genetic markers to separately assess each twin’s DNA can address this, and these same techniques can determine zygosity (whether the twins are identical or fraternal) with high accuracy.
A vanishing twin, where one twin stops developing early in pregnancy, can also create confusing results. The DNA from the nonviable twin may still circulate in the mother’s blood and trigger a false positive.
When Each Test Can Be Done
NIPT is typically offered starting at 10 weeks of gestation, which is the earliest point at which fetal fraction is reliably high enough for accurate results. Some newer assays are being tested as early as 6 weeks, but so far these have only been validated for determining fetal sex, not for chromosomal screening.
CVS is performed between 10 and 13 weeks, making it the earliest available diagnostic option. Amniocentesis is usually done between 15 and 20 weeks. Earlier amniocentesis is possible but associated with higher complication rates and a small number of false negatives, so most providers wait until at least 15 weeks.
Advanced Genetic Testing for High-Risk Pregnancies
When standard chromosome testing doesn’t explain structural abnormalities seen on ultrasound, prenatal exome sequencing can dig deeper. This test reads the protein-coding portions of the baby’s entire genome, looking for mutations in individual genes rather than whole-chromosome problems. In pregnancies where ultrasound findings suggest a genetic condition but standard testing comes back normal, exome sequencing identifies a diagnosis in about 43% of cases. When a diagnosis is made prenatally, it accurately predicts the baby’s condition after birth in the vast majority of cases. This type of testing is typically reserved for pregnancies with significant ultrasound abnormalities and is interpreted by genetics specialists.