Myriad’s Prequel Prenatal Screen reports fetal sex with high accuracy, generally above 99% when performed after 10 weeks of pregnancy. The test analyzes fragments of fetal DNA circulating in the mother’s blood using whole-genome sequencing, which can detect the presence or absence of Y chromosome material. While errors are uncommon, they do happen, and certain biological situations make incorrect results more likely.
How the Test Determines Sex
The Prequel screen is a type of noninvasive prenatal test, or NIPT. It works by drawing a standard blood sample from the pregnant person and analyzing the cell-free DNA (cfDNA) floating in the bloodstream. During pregnancy, the placenta sheds tiny fragments of fetal DNA into the mother’s circulation. The lab sequences all of this DNA and uses computational methods to distinguish fetal fragments from maternal ones.
For sex determination, the process is straightforward: the lab looks for Y chromosome sequences. If Y chromosome DNA is present above a certain threshold, the result comes back male. If it’s absent, the result is female. The test can be performed as early as eight weeks into pregnancy, though accuracy improves as the pregnancy progresses because the concentration of fetal DNA in the mother’s blood increases over time. Results typically take 7 to 10 days after the lab receives the sample.
Where Errors Come From
When sex results are wrong, it’s rarely a lab mistake. The most common causes are biological situations that confuse the DNA signal. Understanding these can help you assess how much confidence to place in your result.
Vanishing twin: If a pregnancy originally involved twins but one stopped developing early on, the placenta from the lost twin may still release DNA into the mother’s blood for weeks afterward. If the vanishing twin was male and the surviving baby is female, the test could incorrectly report a boy.
Low fetal DNA fraction: Early in pregnancy, or in people with a higher body mass, the proportion of fetal DNA in the blood sample can be quite low. When there isn’t enough fetal DNA to analyze reliably, the test may fail to detect Y chromosome fragments, potentially reporting a girl when the baby is actually a boy. Most labs set a minimum threshold and will flag samples that fall below it, but borderline cases can slip through.
Blood transfusion from a male donor: If you received a blood transfusion from a male donor within about four weeks before the test, residual male DNA from that donor blood can register as Y chromosome material, leading the test to report male regardless of the baby’s actual sex.
Organ or bone marrow transplant: Similarly, if you’ve received an organ or bone marrow transplant from a male donor, that transplanted tissue continuously releases male DNA into your bloodstream. This can cause a female fetus to be identified as male.
Placental mosaicism: The fetal DNA in your blood actually comes from the placenta, not the fetus directly. In about 1 to 2% of pregnancies, the placenta’s genetic makeup differs from the baby’s. This is called confined placental mosaicism, and while it more commonly affects chromosome count (like in Turner syndrome), it can occasionally lead to discordant sex results.
Maternal chromosome variations: The test assumes the mother has a standard XX chromosome pattern. Some women carry undiagnosed sex chromosome differences, such as an extra X chromosome, without obvious symptoms. As women age, they also naturally lose X chromosomes in some cells. These variations can interfere with the algorithm that separates maternal from fetal DNA signals.
Undetected maternal cancer: Tumors release their own DNA fragments into the bloodstream. In rare cases, tumor-derived DNA with chromosomal abnormalities can alter the test’s readings, producing unexpected results for both sex determination and chromosome screening.
Timing Matters for Accuracy
Although Myriad states the Prequel screen can be done as early as eight weeks, sex determination tends to be more reliable closer to 10 or 12 weeks. The reason is simple: fetal DNA makes up a larger share of the total cell-free DNA as pregnancy progresses. At eight weeks, fetal DNA might represent only 3 to 4% of the cfDNA in the sample. By 12 weeks, that fraction often doubles. A higher fetal fraction gives the sequencing algorithm more data to work with, reducing the chance of a misread.
If your test was drawn very early and the result surprises you, the timing alone could be a factor. Some providers will recommend a repeat draw a few weeks later if the fetal fraction was borderline.
What a Sex Result Does and Doesn’t Tell You
The Prequel screen identifies chromosomal sex, meaning whether the fetus carries XX or XY chromosomes. It does not assess anatomical development, which is what an ultrasound evaluates later in pregnancy, typically around 18 to 20 weeks. In the vast majority of cases, chromosomal sex and anatomical sex align, but they are technically measuring different things.
It’s also worth noting that NIPT sex results are considered screening, not diagnostic. If a result is unexpected or if there’s a medical reason to confirm (such as a sex-linked genetic condition in the family), diagnostic testing through amniocentesis or chorionic villus sampling provides a definitive answer from the fetus’s own cells rather than placental DNA fragments.
How It Compares to Ultrasound
Ultrasound-based sex determination at the 20-week anatomy scan is also highly accurate, typically around 95 to 99% depending on the baby’s position and the technician’s experience. The key advantage of NIPT is timing: you can learn the baby’s sex months earlier. The tradeoff is that NIPT reads an indirect signal (placental DNA in maternal blood) rather than visualizing anatomy directly. For most pregnancies without the complicating factors listed above, both methods reach the same conclusion.