Can NIPT Test Be Wrong for Gender? The Real Causes

Yes, an NIPT can be wrong for gender, though error rates are low — around 1% when testing is done after 10 weeks of pregnancy.

When you get that NIPT result naming your baby’s sex, a lot of people treat it as fact. Pink or blue, the test feels definitive. It’s a blood draw, not a squirming baby on a sonogram screen. What could go wrong? Plenty of parents base nursery plans, names, and baby shower themes on NIPT gender results. So when the ultrasound later shows something different — or when the baby arrives and you get a surprise — the disconnect feels bewildering.

NIPT is not a gender reveal gimmick. It’s a screening tool that looks at DNA fragments circulating in your blood. For fetal sex determination, it’s highly accurate — but “highly accurate” is not the same as “infallible.” Knowing why and how errors can happen is useful whether you’re hoping for a girl, a boy, or just want to understand how the test works.

How NIPT Determines Fetal Sex

NIPT scans cell-free DNA in your bloodstream, which includes small fragments from the placenta. The test looks specifically for the presence of a Y chromosome. If the test finds Y-chromosome material, the result comes back as male. If it doesn’t, the result is female.

This seems straightforward, but the Y chromosome is smaller than other chromosomes. Detecting it can be compromised by factors that don’t affect assessment of the other chromosomes, according to the genetics lab Sonic Genetics. Low fetal fraction, very early testing, or technical limitations can sometimes mask or mimic Y-chromosome signals.

What’s Actually Being Tested

The DNA comes from the placenta, not from the fetus directly. That distinction matters more than most people realize. The placenta and the fetus start from the same fertilized egg, but sometimes their genetic makeup diverges. When that happens, the placenta’s DNA can tell a different story than the baby’s own chromosomes.

Why People Assume NIPT Is Infallible

NIPT marketing tends to emphasize its high accuracy, often using phrases like “greater than 99% for gender.” Many parents also compare it to ultrasound, which at 11-12 weeks is correct about 87.6% of the time for reporting a male and 96.8% for a female, according to a study in PMC. Because NIPT outperforms ultrasound, it’s easy to assume the blood test is bulletproof.

Another factor is emotional investment. Once you’ve announced the gender, picked a name, and bought a few pink or blue onesies, the psychological stakes climb. The idea that the test could be wrong feels impossible because the social fallout would be so awkward. That hope does not change the biology.

• Confined placental mosaicism: Cells in the placenta have a different chromosomal makeup than the fetus. The test sees the placenta’s DNA and reports that, which may not match the baby.
• Vanishing twin syndrome: A twin stopped developing early but continues to release DNA. The Y chromosome from the vanished twin can trigger a false male result even if the surviving twin is female.
• Maternal DNA abnormalities: Rarely, the mother’s own genetic material includes Y-chromosome sequences from a prior pregnancy or from her own genetics, confusing the test.
• Early testing: If drawn before 10 weeks, the fetal fraction may be too low for reliable detection. The accuracy rate climbs after that window.
• Lab or sample error: Human or technical mistakes are uncommon but not zero. Sample mix-ups, contamination, or data analysis glitches can produce wrong results.

Each of these scenarios is individually rare. But collectively, they explain why roughly 1 out of every 100 NIPT gender results may be incorrect. For most families, that’s still very reliable — but it’s not a guarantee.

When the Y Chromosome Defies the Test

A false male result can happen when the test detects Y-chromosome material that is not coming from the baby. The most common hidden source is a vanishing twin — an early pregnancy loss of one twin that releases DNA into the mother’s bloodstream for weeks afterward. A 2018 study in PubMed followed 57 vanishing twin pregnancies and tracked how often vanishing twin DNA led to discordant NIPT results. The VANISH study, published by Natera, reported 100% fetal sex accuracy for viable twins using that specific test brand. Still, the general principle holds: a demised twin can leave a genetic fingerprint long after the twin is gone.

The reverse — a false female result — can happen if the fetal fraction is too low to detect the Y chromosome at all. If the baby is a boy but the test cannot find enough Y-chromosome fragments, it returns a female result. This is more likely with early drawing or maternal obesity, which can dilute the fetal DNA concentration.

These causes are well-documented in peer-reviewed literature, but they are also uncommon. For most pregnancies, the NIPT gender result matches what the ultrasound and the baby later confirm.

Factors That Raise the Risk of a Wrong Result

1. Testing before 10 weeks: The fetal fraction may be too low, making Y-chromosome detection unreliable. Accuracy improves steadily after this point.
2. Twin or multiple pregnancy: NIPT’s accuracy for fetal sex is not well established in twin pregnancies, according to a PubMed study. A twin that vanishes early can also confuse the result.
3. Maternal factors: Some women have low-level Y-chromosome sequences in their own blood from a prior male pregnancy. Most tests can filter this out, but edge cases exist.
4. Placental mosaicism: When the placenta and the baby have different genetics, the NIPT reads the placenta. This is the most clinically relevant cause of discordant results.

If your NIPT result surprises you or if a later ultrasound contradicts it, your provider can discuss next steps. Repeat testing or an invasive test like amniocentesis or CVS can clarify the baby’s actual chromosomes.

What About Twin Pregnancies and Complex Cases

Most NIPT research has focused on singleton pregnancies. For twins, the evidence is thinner. A 2019 PubMed study noted that NIPT is accurate for fetal sex in singletons but explicitly stated its accuracy “is not well established in twin pregnancies.” That’s a significant gap, especially because twin pregnancies have higher rates of vanishing twin syndrome and confined placental mosaicism.

The placenta is a living tissue, and its cells can have chromosomal differences from the fetus. That condition, called confined placental mosaicism, is one of the most well-documented causes of discordant NIPT results. When CPM is present, the test may report a wrong sex or even flag a false positive for a chromosomal condition. More research is needed to increase NIPT’s sensitivity for detecting mosaicism, as noted in a peer-reviewed Frontiers in Medicine article.

If you are carrying twins, or if you had a vanishing twin early in your pregnancy, your genetic counselor or obstetrician may recommend additional monitoring or a confirmatory test before making decisions based on the NIPT gender result.

The Bottom Line

NIPT is a strong screening tool for fetal sex, but it is not 100% perfect. About 99% of results are correct after 10 weeks. Errors happen through confined placental mosaicism, vanishing twin DNA, low fetal fraction, and rare lab mishaps. Most of the time the result is right, but knowing the possible pitfalls can help you avoid a big surprise later.

If you have a vanishing twin history, are carrying multiples, or simply want absolute certainty before planning a gender reveal, your obstetrician or genetic counselor can walk through whether an ultrasound follow-up or invasive testing makes sense for your specific pregnancy.