The MaterniT21 test is 99.4% accurate for determining fetal sex, making it one of the most reliable ways to find out whether you’re having a boy or girl. Results are available as early as 9 weeks into pregnancy, well before a standard anatomy ultrasound at 18 to 20 weeks.
How the Test Determines Sex
MaterniT21 is a type of noninvasive prenatal test (NIPT) that analyzes small fragments of your baby’s DNA circulating in your bloodstream. Starting around week 9 of pregnancy, enough of this fetal DNA is present for the lab to detect whether a Y chromosome is there. If Y chromosome sequences show up, the result is male. If they don’t, the result is female.
This is fundamentally different from an ultrasound, which relies on a technician visually identifying anatomy. The DNA-based approach is why the accuracy rate is so high and why results are available months earlier than ultrasound-based sex determination.
What 99.4% Accuracy Means in Practice
A 99.4% sensitivity means that out of every 1,000 pregnancies tested, roughly 6 could receive an incorrect sex result. For most people, those odds are reassuring enough to confidently plan around the result. But it’s worth understanding that “nearly perfect” is not the same as “guaranteed,” especially if you’re making decisions like purchasing a large amount of gendered items or, in rarer cases, when sex determination matters for sex-linked genetic conditions.
The accuracy for sex determination is actually on par with the test’s accuracy for detecting chromosomal conditions like Down syndrome, which is what the test was originally designed for. Sex reporting is included as part of the standard panel, not as a separate add-on.
When Results Can Be Wrong
Several specific situations can throw off a sex result, even with a test this accurate.
- Low fetal fraction. If the percentage of your baby’s DNA in your blood sample is too low, the lab may not detect a Y chromosome even when one is present. This is more common in earlier gestational ages and in people with a higher BMI, since more maternal DNA dilutes the fetal signal. When fetal fraction is too low, the lab typically flags the sample as inconclusive rather than giving a wrong answer, and you’ll be asked for a redraw.
- Vanishing twin. If a twin pregnancy was present early on but one embryo stopped developing, DNA from that lost twin can linger in your bloodstream for 8 weeks or more. If the vanished twin was a different sex than the surviving baby, the test may pick up conflicting signals. Studies have found that vanishing twins account for 15% to 33% of false-positive NIPT results overall.
- Sex chromosome conditions. Conditions like Turner syndrome (where a girl has only one X chromosome) or Klinefelter syndrome (where a boy has an extra X) can complicate sex reporting. The detection rate for sex chromosome conditions is around 95%, which is lower than the test’s accuracy for standard sex determination or for conditions like trisomy 21. False positives for these conditions do occur.
For a straightforward singleton pregnancy with adequate fetal fraction, the chance of getting the wrong sex result is very small.
Timing and Results
The test can be performed starting at 9 weeks of gestation. Your provider draws a standard blood sample from your arm, which is sent to Labcorp for processing. Results typically come back in 3 to 5 business days, though your provider’s office may take additional time to review and relay them.
In practice, many providers order the test between weeks 10 and 12. Waiting a bit beyond the 9-week minimum can increase the amount of fetal DNA in your blood, reducing the chance of an inconclusive result that requires a second draw. If a redraw is needed, you’ll usually wait another week or two before repeating the test.
Twin and Multiple Pregnancies
Sex determination becomes less straightforward with multiples. In a twin pregnancy, if the test detects a Y chromosome, it confirms at least one baby is male, but it can’t tell you whether both are male or just one. If no Y chromosome is detected, both babies are likely female.
For higher-order multiples (triplets or more), ACOG does not recommend cell-free DNA screening due to insufficient data on reliability. The same limitation applies to screening for sex chromosome conditions in twin pregnancies.
How It Compares to Ultrasound
A skilled ultrasound technician can determine sex with roughly 95% to 99% accuracy at the 18- to 20-week anatomy scan, depending on the baby’s position. The main advantage of MaterniT21 isn’t a dramatically higher accuracy rate for sex. It’s the timing. Getting a reliable answer at 10 weeks rather than 20 weeks is the practical difference most people care about.
If your MaterniT21 result and your later ultrasound disagree on sex, that discrepancy is worth flagging to your provider. It could simply be an ultrasound positioning issue, but in rare cases it can signal a vanishing twin, a sex chromosome condition, or another finding that warrants a closer look.