How Accurate Is the NIPT Test for Twins’ Gender?

NIPT is highly accurate at detecting fetal sex in twin pregnancies, though its reliability depends on the specific combination of sexes involved. When both twins are girls, the test achieves near-perfect results. When at least one boy is present, accuracy remains strong but drops slightly, and the test has a harder time distinguishing between a pair of boys and a boy-girl pair. The picture gets more complicated when a pregnancy started as twins but one was lost early on, a scenario that can leave misleading DNA fragments in the mother’s blood for weeks.

How NIPT Reads Fetal Sex in a Twin Pregnancy

NIPT works by analyzing fragments of fetal DNA circulating in the mother’s blood. These fragments come from the placenta and mix with the mother’s own cell-free DNA. In a singleton pregnancy, sex determination is straightforward: if Y-chromosome DNA shows up, the baby is male; if none is detected, the baby is female. In a twin pregnancy, the logic gets trickier. Both placentas shed DNA into the same maternal bloodstream, so the test is working with a mixed signal. If any Y-chromosome material appears, the lab knows at least one twin is male. If none appears, both are likely female. But figuring out whether Y-chromosome DNA is coming from one fetus or two requires more sophisticated analysis, and that is where accuracy can vary.

Different NIPT platforms handle this challenge in different ways. Some use whole-genome sequencing to count chromosomes and estimate relative proportions of X and Y material. Others use a method based on single-nucleotide polymorphisms, which can distinguish individual genetic profiles more precisely. The approach matters because it affects how well the test can separate the DNA contributions of each twin, especially when trying to determine whether one or both are male.

Accuracy When Both Twins Are Girls

The simplest scenario for NIPT is a pregnancy with two female fetuses. Because girls have no Y chromosome, the test only needs to confirm the absence of Y-chromosome DNA in the mother’s blood. A study analyzing 198 twin pregnancies found that the prediction model achieved 100% sensitivity and 100% specificity when both twins were female.¹ In practice, this means no female-female pregnancies were misidentified as having a male fetus, and no male-containing pregnancies were misidentified as all-female.

This high accuracy makes intuitive sense. Detecting the absence of something is relatively simple when your test is sensitive enough. As long as the fetal DNA fraction in the mother’s blood is adequate and no contamination from a prior or vanished pregnancy is present, the all-female result is the most reliable sex call NIPT can make in twins.

When at Least One Twin Is Male

Once Y-chromosome DNA appears in the sample, the test confidently identifies that at least one male fetus is present. That same study of 198 twin pregnancies reported 98% sensitivity and 95% specificity for detecting the presence of a male twin.¹ The slight dip from the perfect female-female numbers reflects the added complexity of interpreting Y-chromosome signal levels. A small number of cases may produce borderline readings where Y-chromosome material is detected at very low levels, creating ambiguity about whether it represents a true signal or background noise.

The real difficulty comes when the test tries to go a step further and determine whether both twins are boys or just one of them. An earlier study focused specifically on this distinction found that while it could perfectly separate “at least one male” pregnancies from all-female pregnancies, telling apart two-boy pregnancies from boy-girl pregnancies was far less reliable, achieving only about 76% accuracy.² The reason is that the concentration of male DNA in the mother’s blood overlaps between these two groups. Two male placentas shed more Y-chromosome material than one, on average, but the ranges overlap enough that a clear cutoff is hard to set.

Newer Methods That Can Identify Each Twin Individually

The 76% figure for distinguishing two boys from a mixed-sex pair came from an approach that relied primarily on measuring the quantity of Y-chromosome DNA. More recent NIPT platforms using single-nucleotide polymorphism analysis take a different approach: instead of just measuring how much Y-chromosome DNA is present, they analyze genetic variation patterns to distinguish the DNA of one twin from the other. A validation study using this method reported 100% accuracy for individual fetal sex calls across 102 twin fetuses.³

This represents a meaningful leap. Instead of saying “at least one of your twins is a boy,” the test can say “Twin A is a boy and Twin B is a girl” with high confidence. The catch is that not all commercial NIPT platforms offer this level of detail, and not all labs use the same technology. If you are pregnant with twins and want individual sex results rather than a grouped answer, it is worth asking your provider which platform they use and whether it can resolve individual fetal profiles.

NIPT Compared to First-Trimester Ultrasound

Many parents first learn about fetal sex through ultrasound, so a natural question is how NIPT stacks up against imaging. In twin pregnancies, NIPT has a clear edge early on. A prospective study of 113 women with twin pregnancies found that cell-free DNA testing achieved 100% sensitivity and specificity for Y-chromosome detection, while ultrasound-based sex assignment during first-trimester scans was correct only about 80% of the time.⁴

Ultrasound accuracy improved as the fetuses grew, climbing from roughly 54% at the smallest fetal sizes to about 92% at larger sizes still within the first trimester. But even at its best, first-trimester ultrasound was less reliable than the blood test. Male fetuses were particularly prone to being misidentified by ultrasound, with odds of an error more than 20 times higher compared to female fetuses.⁴ By the second trimester, ultrasound accuracy improves substantially, which is why many providers wait until around 18 to 20 weeks for a confident visual sex determination. But for parents eager to know earlier, NIPT is the more dependable route in twin pregnancies.

The Vanishing Twin Problem

One of the most significant sources of wrong sex results in twin NIPT has nothing to do with the test’s technology and everything to do with biology. Vanishing twin syndrome occurs when one of the embryos in a multiple pregnancy stops developing and is gradually reabsorbed. This happens more often than most people realize, particularly in pregnancies conceived through assisted reproduction. The trouble is that the demised twin’s placental tissue can continue releasing DNA into the mother’s bloodstream for weeks after the loss.

The most common way this affects sex results is when a male twin is lost early in pregnancy. Y-chromosome DNA from the vanished twin persists in the mother’s blood, and the NIPT picks it up. The surviving twin may be female, but the test reports the presence of a male because it is detecting real Y-chromosome material from what is essentially a ghost signal.⁵ This type of sex discordance is the single most frequent way vanishing twins affect NIPT results.

Research on how long this contamination lasts suggests the vanished twin’s DNA influence persists for at least seven to eight weeks but typically fades within 12 to 14 weeks during the first and second trimesters.⁶ A larger study comparing vanishing twin pregnancies with singleton pregnancies found that vanishing twin cases had a sex discrepancy rate of about 10%, compared to just 0.05% in ordinary singleton pregnancies. However, when testing was performed after 14 weeks of gestation, the discrepancy rate dropped to roughly 8%, and in several cases where the test was repeated at a later date, the contaminating DNA had faded enough to produce a correct result.⁷

If you know or suspect you had a vanishing twin, this is important context for interpreting your NIPT sex result. Some clinicians recommend delaying NIPT or repeating it after 14 weeks in these cases. Others suggest treating the sex result with extra caution and confirming with a later ultrasound.

Maternal Factors That Can Cause False Results

Vanishing twins are not the only source of misleading sex results. The mother’s own genetics can occasionally interfere. A systematic analysis of false-positive sex chromosome results from nearly 24,000 NIPT cases found that maternal X-chromosome abnormalities, small structural variations in the mother’s DNA, and placental mosaicism all contributed to incorrect calls.⁸ These are uncommon individually, but they add up. In that study, about half of all false-positive sex chromosome results could be traced to a maternal or placental origin rather than a true fetal abnormality.

Placental mosaicism deserves a brief mention because it is relevant to how NIPT works. The DNA fragments the test analyzes come from the placenta, not directly from the fetus. In rare cases, the placenta’s genetic makeup does not perfectly match the fetus. If the placenta has cells with a different sex chromosome complement than the fetus itself, NIPT will report the placenta’s genetics, not the baby’s. This is uncommon enough that it does not meaningfully dent the overall accuracy numbers, but it is worth knowing about because it means NIPT is technically a placental test, not a fetal test.

What Happens When NIPT and Ultrasound Disagree

In most twin pregnancies, NIPT and the anatomy ultrasound will agree on fetal sex, and the question never comes up. But when they disagree, it creates real anxiety. The clinical recommendation in these situations is a careful, stepwise evaluation. The first thing providers typically rule out is simple human error: mislabeled samples, mixed-up results between patients, or clerical mistakes. These account for a meaningful share of apparent discrepancies.

Once human error is eliminated, the disagreement may point to something biologically interesting and occasionally medically important. When NIPT says male and ultrasound shows female anatomy (or vice versa), it can indicate a disorder of sex development, a chromosomal condition, or the vanishing twin scenario described above. Clinical guidelines recommend that families in this situation be counseled by specialists with experience in both genetics and disorders of sexual development, so that appropriate evaluation and preparation can begin before birth.⁹

For twin pregnancies specifically, a discrepancy is somewhat more likely to have a benign explanation than in a singleton pregnancy, simply because the DNA signal is more complex. Two placentas contributing to the same blood sample leaves more room for ambiguous readings. If your NIPT result and your ultrasound do not match, it is not necessarily a sign of a medical problem, but it is a reason to follow up with your provider rather than assume one result is right and the other is wrong.

Identical vs. Fraternal Twins and Why It Matters

Whether your twins are identical or fraternal affects what NIPT can tell you. Identical twins share the same DNA and the same sex, so a single sex call applies to both. The test does not need to distinguish between two separate DNA profiles, which simplifies the analysis. Fraternal twins, on the other hand, are genetically distinct siblings who happen to share a womb. They can be the same sex or different sexes, and their placentas shed different DNA into the mother’s blood. This is where the challenge of separating individual profiles becomes relevant.

Some NIPT platforms can also determine zygosity, meaning they can tell you whether your twins are identical or fraternal. The same SNP-based method that achieved 100% accuracy for individual sex determination in twin fetuses was also able to assess zygosity.³ Knowing zygosity early can be clinically useful beyond sex determination: identical twins who share a placenta face specific risks like twin-to-twin transfusion syndrome that require closer monitoring.

Low Fetal Fraction and Failed Tests

Sometimes NIPT does not return a sex result at all because the fetal DNA fraction in the mother’s blood is too low. Twin pregnancies generally produce more fetal DNA than singletons because two placentas are contributing, but the per-fetus fraction can still be lower than in a singleton pregnancy. Factors like early gestational age, higher maternal body weight, and certain placental characteristics can push the fetal fraction below the threshold needed for a reliable result. When this happens, the lab typically reports a “no call” or requests a repeat blood draw at a later date. A failed draw is not the same as an inaccurate result; it just means the test could not generate enough confidence to make a prediction.

If your NIPT comes back without a sex result, the usual recommendation is to repeat the test a week or two later, when the fetal fraction will likely be higher. In the vast majority of cases, the repeat draw produces a usable result. Persistently low fetal fraction across multiple draws is rare but can itself be a clinical finding worth discussing with your provider, as it occasionally correlates with placental issues.

Practical Considerations for Parents of Twins

If your main goal is to learn whether you are having boys, girls, or one of each, NIPT performed after 10 weeks of gestation will give you a reliable answer in the large majority of cases. The all-female result is the most trustworthy. A result showing at least one male is very likely correct but slightly less certain, and whether the lab can tell you the sex of each individual twin depends on the platform used. If you conceived through IVF or know that a twin was lost early in pregnancy, mention this to your provider before interpreting results, because vanishing twin DNA can linger and produce a misleading male signal.

Ultrasound remains the standard confirmation tool by the second trimester, and most providers will verify NIPT sex findings at the anatomy scan around 18 to 20 weeks. For most twin pregnancies, the two methods will agree, giving you high confidence in the result. When they do not, it is a conversation starter with your care team rather than a cause for alarm.