MaterniT21 is highly accurate for detecting Down syndrome, with a sensitivity of 100% and a specificity above 99% in clinical validation studies. That means it catches virtually every case and produces very few false alarms. But “highly accurate” doesn’t mean it’s a diagnostic test, and the numbers shift depending on what condition it’s screening for, your age, and your weight.
Detection Rates for Common Trisomies
MaterniT21 screens for three major chromosomal conditions by analyzing fragments of placental DNA circulating in your blood. For the primary conditions it targets, the numbers are strong but not identical across the board.
For trisomy 21 (Down syndrome), the test achieved 100% sensitivity in validation studies, with a false positive rate of just 0.1% and a positive predictive value of 97.9%. That positive predictive value means that when the test flags a pregnancy as high risk for Down syndrome, it’s correct roughly 98 times out of 100.
For trisomies 18 (Edwards syndrome) and 13 (Patau syndrome), there were no false positives in validation data, giving a positive predictive value of 100%. However, the test did miss 3 cases of trisomy 18 and 2 cases of trisomy 13, so its sensitivity for these rarer conditions is slightly lower than for Down syndrome.
How It Compares to Traditional Screening
The gap between MaterniT21 and older screening methods is substantial. First trimester screening (the combination of blood work and a nuchal translucency ultrasound) detects about 85% of Down syndrome cases, with a false positive rate around 5%. The quad screen, done in the second trimester, catches about 80% with the same 5% false positive rate.
Cell-free DNA screening like MaterniT21 detects 99% of cases with a false positive rate of 0.1 to 0.2%. That’s a 25- to 50-fold reduction in false positives compared to traditional screening, which means far fewer families face the anxiety of a false alarm and the pressure to decide about follow-up invasive testing.
Why a Positive Result Isn’t a Diagnosis
Even with 99%+ accuracy, MaterniT21 is still a screening test. The critical number here is the positive predictive value, and it changes based on how common the condition is in your specific risk group. A 35-year-old has a higher baseline risk for Down syndrome than a 25-year-old, so a positive result at 35 is more likely to be a true positive. For younger women with lower baseline risk, a positive result has a higher chance of being a false alarm, even with the same test performance.
The American College of Obstetricians and Gynecologists is clear on this: a positive cell-free DNA result should be followed by genetic counseling and a recommendation for diagnostic testing with chorionic villus sampling (CVS) or amniocentesis. These procedures analyze fetal cells directly and give a definitive yes or no. No irreversible decisions should be made based on a screening result alone.
Sex Chromosome Conditions Are Less Reliable
MaterniT21 also screens for sex chromosome conditions, but the accuracy drops compared to the major trisomies. Detection of Turner syndrome (monosomy X) showed sensitivity above 99%, though that figure came from a small sample of just 10 cases. For Klinefelter syndrome (XXY), sensitivity was only 80%, meaning one in five cases could be missed.
Fetal sex determination, on the other hand, is very reliable. The test correctly identified male fetuses more than 99.9% of the time and female fetuses 99.3% of the time in validation testing with over 300 samples.
When the Test Can’t Give a Result
MaterniT21 works by measuring tiny fragments of placental DNA floating in your bloodstream, called fetal fraction. If there isn’t enough of this DNA relative to your own, the lab can’t produce a reliable result. This is called a “no call” outcome.
Overall, this happens in fewer than 1 in 200 tests (0.48%). But maternal weight is the biggest factor driving no-call rates. For women under 150 pounds tested between 9 and 12 weeks, the failure rate was just 0.14%. For women over 400 pounds at the same gestational age, it jumped to 17.39%. The likelihood of a test failure has been reported at greater than 10% for women over 250 pounds and above 50% for women at or above 350 pounds.
The reason is biological: higher body weight increases the amount of maternal DNA in the blood while diluting the relative concentration of placental DNA. Waiting a few extra weeks helps, since fetal fraction rises as pregnancy progresses. At 13 to 18 weeks, the no-call rate for women over 400 pounds dropped to 6.45%. In the highest weight category, fetal fraction increased about 0.5% with each additional week of pregnancy.
A no-call result isn’t the same as a negative. ACOG recommends that patients who receive a nonreportable result be offered genetic counseling, a comprehensive ultrasound, and diagnostic testing, because no-call results are associated with a higher risk of fetal aneuploidy.
Twin Pregnancies
Cell-free DNA testing in twins has historically had less data behind it, but a large multicenter study found that MaterniT21 demonstrated high sensitivity and specificity for trisomy 21 in twin pregnancies, comparable to what’s seen in singleton pregnancies. Performance for trisomies 13 and 18 in twins was described as promising but based on more limited data.
Timing and Results
The test can be drawn as early as 9 weeks of gestation. Results typically take 7 to 14 days. Drawing too early increases the chance of a low fetal fraction and a no-call result, particularly if you have a higher BMI. If your provider suggests waiting an extra week or two, that delay can meaningfully improve the odds of getting a usable result on the first draw.