MaterniT21 is highly accurate for predicting fetal sex, with sensitivity above 99% in most validated studies and specificity near 99% as well. For the vast majority of singleton pregnancies, the result you receive will match your baby’s actual sex. The test can be performed as early as nine weeks of gestation, making it one of the earliest reliable ways to find out.
Overall Accuracy Numbers
A large meta-analysis of cell-free DNA tests (the technology behind MaterniT21) found an overall sensitivity of 96.6% and specificity of 98.9% for fetal sex determination. Sensitivity refers to how well the test correctly identifies male fetuses, while specificity measures how well it correctly identifies female fetuses. More recent analytic validations have reported correct sex assignment in 99% to 100% of cases.
Accuracy improves slightly as pregnancy progresses. In the first trimester, sensitivity sits around 95% and specificity around 98.8%. By the second trimester, those numbers climb to 98.2% sensitivity and 99.5% specificity. This improvement happens because the amount of fetal DNA circulating in your blood increases as the placenta grows. Still, even at nine or ten weeks, the test is remarkably reliable for a simple blood draw.
How the Test Works
MaterniT21 analyzes fragments of DNA from the placenta that circulate freely in your bloodstream. To determine sex, it looks for the presence or absence of Y chromosome material. If Y chromosome DNA is detected above a certain threshold, the result is reported as male. If it’s absent, the result is female. The test requires a minimum gestational age of nine weeks, because before that point there may not be enough fetal DNA in your blood to produce a reliable reading.
What Can Cause a Wrong Result
Incorrect sex predictions are rare, but they do happen. Labcorp, which manufactures MaterniT21, lists several biological reasons a result might be wrong:
- Vanishing twin. If a twin pregnancy was present early on but one embryo stopped developing, leftover DNA from that twin can still circulate in your blood. If the vanishing twin was male and the surviving baby is female, the test may incorrectly report male.
- Mosaicism. Sometimes the placenta, the fetus, or even the mother’s own cells carry a mix of different genetic patterns. If the placenta has some cells with a Y chromosome and others without, the test can be thrown off.
- Prior organ transplant. If you’ve received an organ from a male donor, donor DNA circulating in your blood could be mistaken for fetal Y chromosome material.
- Low fetal fraction. If the amount of fetal DNA in your blood is unusually low (which can happen with higher BMI or very early testing), the lab may not have enough material for a confident call. In many of these cases the lab will flag the sample as inconclusive rather than give a wrong answer.
Accuracy in Twin Pregnancies
Sex determination gets more complicated with twins. For identical twins sharing a single placenta, the test performs extremely well, reaching 100% sensitivity and specificity in one study for distinguishing two girls from two boys. That makes sense: identical twins are virtually always the same sex, so the test only needs to detect whether Y chromosome DNA is present or not.
Fraternal twins with separate placentas are harder. The challenge is distinguishing a boy-girl pair from two boys, since both scenarios produce Y chromosome DNA in your blood. One study found that a basic analysis correctly identified mixed-sex pairs with about 83% sensitivity, but a refined two-step method pushed that to 98% sensitivity. Even with improved methods, researchers noted that unequal DNA shedding from the two placentas can create enough imbalance to produce a wrong call. Confirmation by ultrasound is recommended for twin pregnancies.
How It Compares to Ultrasound
Before cell-free DNA testing existed, ultrasound was the standard way to learn your baby’s sex. A skilled sonographer can typically determine sex with high accuracy at the 18 to 20 week anatomy scan by visualizing the external genitalia. Earlier ultrasounds (around 12 to 13 weeks) can also attempt sex determination, but accuracy drops because the anatomy is still developing.
The main advantage of MaterniT21 is timing. You can get a result as early as nine or ten weeks, roughly two months before the anatomy scan. The accuracy is comparable to or slightly better than ultrasound, and it doesn’t depend on the baby being positioned in a way that lets the sonographer get a clear view. For parents who want to know early, cell-free DNA testing is the most reliable option available at that stage of pregnancy.
Sex Chromosome Conditions
MaterniT21 also screens for conditions involving extra or missing sex chromosomes, such as Turner syndrome (a single X chromosome) or Klinefelter syndrome (XXY). Detection rates for these conditions sit around 95%, which is lower than the test’s accuracy for common chromosome conditions like Down syndrome. If the test flags a possible sex chromosome condition, it can sometimes complicate the sex prediction itself. These findings are always considered screening results, not diagnoses, and follow-up testing is typically offered to confirm.
If your MaterniT21 results come back with a clear sex call and no flags for chromosome conditions, the prediction is correct in the vast majority of cases. The rare errors almost always trace back to one of the biological scenarios listed above rather than a flaw in the technology itself.