A girl can be mistaken for a boy on a 20-week ultrasound, though the error is uncommon. Studies consistently report accuracy rates above 99 percent for sex determination in the second trimester, but that still leaves a small number of wrong calls, and the mistake tends to run in one direction: girls misidentified as boys. The reasons range from an awkward fetal position to rare genetic conditions that genuinely change how the genitalia look, and understanding why errors happen can help you know when to trust the prediction and when to press for a closer look.
How Accurate Is a 20-Week Ultrasound for Determining Sex?
The 20-week anatomy scan is the standard point in pregnancy when most parents learn their baby’s sex. By this stage the external genitalia are usually well-developed enough to see clearly. In one study of more than 800 pregnancies, sonographers were able to assign a sex in about 93 percent of cases, and among those assignments the overall accuracy was above 99 percent.1PubMed. Early second-trimester determination of fetal gender by ultrasound A separate study found that sex assignment was possible in all but one out of 215 second-trimester scans, with 100 percent accuracy in the cases where a prediction was made.2PubMed Central. Accuracy of sonographic fetal gender determination: predictions made by sonographers during routine obstetric ultrasound scans So the odds are strongly in favor of a correct answer. But “strongly in favor” is not the same as guaranteed, and the errors that do occur follow a pattern worth knowing about.
Why Girls Get Misidentified as Boys More Often Than the Reverse
When mistakes happen, they are more likely to involve a female fetus being called male. Research has found that errors in sex determination were more common when assessing a female fetus than a male one.1PubMed. Early second-trimester determination of fetal gender by ultrasound The reason comes down to what a sonographer is looking for. Identifying a boy usually means spotting the penis and scrotum, structures that tend to stand out. Identifying a girl means confirming the absence of those structures and seeing the labia, which are smaller and subtler.
Several things can mimic a male appearance on ultrasound when the fetus is actually female. The umbilical cord can drape between the legs and cast a shadow that looks like a penis. Swollen labia, which are normal in developing female fetuses because of circulating hormones, can be mistaken for a scrotum. A clenched hand resting near the groin can also create a confusing image. In a small 3D ultrasound study focused on the mid-sagittal plane, female assignment accuracy dropped when image quality was poor, with two of five incorrect male assignments in the 19-to-24-week window flagged as doubtful by the sonographer at the time. When those doubtful cases were excluded, accuracy for female assignment returned to 100 percent.3PubMed. Three-dimensional multiplanar ultrasound for fetal gender assignment: value of the mid-sagittal plane The takeaway: when a sonographer expresses confidence, the call is almost always right. When they hedge, that hedge deserves attention.
How Fetal Position and Maternal Body Type Affect the Call
Even the best sonographer can only read what the ultrasound shows, and sometimes the image quality just isn’t good enough. The fetus might have its legs crossed or pressed tightly together, hiding the genitalia entirely. It might be facing the wrong way. These positional issues are the most common reason a sonographer cannot make a determination at all, rather than making the wrong one. In the study with 215 scans, the single case where sex could not be assigned involved a combination of high maternal BMI, uterine fibroids, and an unfavorable fetal position.2PubMed Central. Accuracy of sonographic fetal gender determination: predictions made by sonographers during routine obstetric ultrasound scans
Maternal weight plays a real role. Ultrasound relies on sound waves traveling through tissue, and more abdominal tissue means a weaker, grainier image. One study found that the ability to adequately visualize all fetal anatomy during a scan dropped sharply with increasing BMI: about 72 percent of women with a normal BMI had all anatomical components seen on the first attempt, compared with just 30 percent of women with the highest obesity classification.4PubMed. Maternal obesity limits the ultrasound evaluation of fetal anatomy Another study similarly concluded that obese women had significantly more sub-optimal views and required follow-up appointments at higher rates.5PubMed. Impact of maternal obesity on procedure of mid-trimester anomaly scan Reduced image quality does not always lead to a wrong sex determination, but it increases the chance of either an uncertain call or an incorrect one.
Sonographer Experience Matters More Than You Might Think
Not all sonographers get it right at the same rate. Research comparing the performance of individual sonographers during routine scans found statistically significant differences in their success rates for making a prediction in the first place.2PubMed Central. Accuracy of sonographic fetal gender determination: predictions made by sonographers during routine obstetric ultrasound scans This doesn’t necessarily mean some sonographers are bad at their jobs. It more likely reflects differences in patience, technique, and willingness to wait for a clear view versus offering a tentative guess. A quick look at a suboptimal angle is where mistakes creep in. If you’re at the 20-week scan and the sonographer says they aren’t sure, that honesty is actually the most reliable thing they can tell you. It’s better to come back for a repeat scan than to receive a prediction that was made under pressure.
Genetic Conditions That Make External Genitalia Ambiguous
Sometimes the ultrasound image is perfectly clear and the sonographer reads it correctly, but the genitalia themselves don’t match the baby’s chromosomes. This is rarer than a simple imaging error, but it’s the scenario most likely to produce real confusion later on.
Congenital adrenal hyperplasia (CAH) is one of the more common causes. In a female fetus with CAH, the adrenal glands overproduce androgens, which can cause the external genitalia to look partially or fully masculinized. The clitoris may enlarge to resemble a penis, and the labia may partially fuse, mimicking a scrotum. On ultrasound this can genuinely look like a boy. Researchers have demonstrated that this virilization can be assessed prenatally using detailed fetal ultrasound, and early prenatal diagnosis is important because treatment before birth can reduce the degree of genital changes.6PubMed. Prenatal diagnosis and in utero treatment of congenital adrenal hyperplasia: An up-to-date comprehensive review One case report showed how the degree of virilization in a female fetus with CAH was assessed at 30 weeks on targeted ultrasound, revealing features like an abnormal urethral course and vaginal changes that pointed to a difference of sex development.7PubMed Central. Prenatal Phenotyping of Fetal Congenital Adrenal Hyperplasia: Applying the Prader Scale to a Fetus
The opposite scenario also exists. Complete androgen insensitivity syndrome (CAIS) involves a fetus with XY chromosomes whose body does not respond to androgens, resulting in typically female-appearing external genitalia despite a male genetic makeup.8PubMed Central. Complete Androgen Insensitivity Syndrome: Role of Imaging for Diagnosis In those cases the ultrasound would show what looks like a girl, and the sonographer would call it a girl. In practical terms that’s not really a “mistaken” call because the external anatomy genuinely appears female. The discrepancy only surfaces if chromosomal testing is done for another reason. One reported case was diagnosed prenatally after cell-free fetal DNA screening flagged a male genetic result that didn’t match what the ultrasound showed.9PubMed Central. Complete Androgen Insensitivity Syndrome: A Rare Case of Prenatal Diagnosis
Another condition that can mislead the ultrasound is hypospadias, where the urethral opening in a male fetus is positioned abnormally. In severe cases the penis curves downward and the genital tubercle points in the direction typically associated with a female fetus. One documented case involved a fetus initially assigned female at 14 weeks based on the angle of the genital tubercle. An amniocentesis later revealed male chromosomes, and a follow-up scan at 22 weeks showed a curved penile shaft consistent with penoscrotal hypospadias.10PubMed. Hypospadias mimicking female genitalia on early second trimester sonographic examination That’s technically a boy being mistaken for a girl rather than the other way around, but it illustrates how genital anomalies can scramble the visual cues that sonographers rely on. Among prenatally imaged genital defects in one study, the most common group was undervirilized boys, accounting for roughly 69 percent of cases and including many with hypospadias.11PubMed. Prenatal imaging of genital defects: clinical spectrum and predictive factors for severe forms
When Cell-Free DNA and Ultrasound Disagree
Cell-free DNA screening, often called NIPT, can determine chromosomal sex from a maternal blood sample as early as 10 weeks. Because it reads the fetal DNA fragments circulating in the mother’s blood, it tells you whether Y-chromosome material is present, which is a different kind of information from what ultrasound provides. Most of the time NIPT and ultrasound agree. But discordant results, where one says male and the other says female, do occur. Research puts the frequency at roughly 1 in 1,500 to 2,000 pregnancies.12Wiley Online Library (Prenatal Diagnosis). Discordant fetal sex on NIPT and ultrasound
A discrepancy between NIPT and ultrasound doesn’t always mean either test was wrong in the usual sense. It can be the first clue to one of the conditions described above, like CAH or CAIS, where the chromosomes and the visible anatomy genuinely don’t match. It can also reflect technical issues: a vanishing twin with different chromosomes, confined placental mosaicism, or simply a lab error. When a discrepancy shows up, the standard recommendation is referral for a detailed expert ultrasound, genetic counseling, and sometimes invasive diagnostic testing to sort out what’s going on.13PubMed. Prenatal ultrasound finding of atypical genitalia: Counseling, genetic testing and outcomes Atypical-looking genitalia on ultrasound, whether or not there’s a NIPT discrepancy, should similarly prompt a more thorough workup.14PubMed. Fetal genital anomalies: an aid to diagnosis
Limitations of Even the Most Advanced Imaging
Parents sometimes assume that if the standard 2D ultrasound is uncertain, a 3D or 4D scan will settle the question. That’s true in many cases, and researchers have explored using 3D imaging and even virtual-reality environments to better evaluate ambiguous genitalia. One research team found that binocular depth perception in a virtual-reality setup improved the ability to distinguish structures like an enlarged clitoris from a micropenis.15PubMed. Using virtual reality for evaluation of fetal ambiguous genitalia But even with advanced technology, the assessment can be genuinely difficult. A recent review concluded that prenatal diagnosis of abnormal external genitalia remains challenging even with 3D ultrasound.16PubMed. Abnormal fetal genitalia: two- and three-dimensional ultrasound assessment Better equipment helps, but it doesn’t eliminate ambiguity when the anatomy itself is unusual.
This matters because parents who receive an uncertain or seemingly wrong sex determination sometimes seek repeated scans at private imaging centers, hoping a higher-resolution picture will give them a definitive answer. For the vast majority of pregnancies where the anatomy is typical, a good look from a skilled sonographer at 20 weeks is genuinely reliable. The cases where the answer stays murky are usually the ones involving a true anatomical variation, and those require genetic testing rather than more pictures.
The Emotional Fallout of a Wrong Call
An incorrect sex prediction might seem like a minor inconvenience, a nursery repainted, some clothes exchanged. But research into the real-world effects suggests the impact can be deeper than that. A study examining the implications of incorrect fetal sex determination found that affected women experienced marital conflicts, negative perceptions of ultrasound technology, and in some cultural contexts, domestic violence. Some women who had undergone tubal ligation based partly on having achieved a desired family composition expressed a desire to reverse the procedure.17PubMed. Implications of incorrect determination of fetal sex by ultrasound
The psychological weight of being told one thing for months and then discovering another at birth can catch parents off guard. Some parents bond with the idea of a son or daughter throughout pregnancy, choose a name, and imagine a specific child. When the baby arrives and the sex is different, the adjustment is usually swift, but the initial shock is real. In settings where sex-selective pressures exist, the consequences can be much more serious. This is one reason many sonographers and clinicians are careful about how they communicate sex predictions, and some institutions have policies about when and how the information is shared.
What to Do If You Suspect the Prediction Is Wrong
If your 20-week scan produced a confident sex prediction and you have no other reason for concern, the call is overwhelmingly likely to be correct. But a few situations warrant a second look. If the sonographer seemed uncertain or said they only got a brief glimpse, asking for a repeat scan at a later appointment is reasonable. If you’ve had NIPT and the result conflicts with the ultrasound, that discrepancy should be flagged to your provider, since it can indicate something beyond a simple imaging mix-up. And if there’s a family history of conditions like CAH or differences of sex development, mentioning that to the sonographer before the scan can help them pay closer attention to the genital anatomy.
For most parents, though, the 20-week anatomy scan provides a reliable answer. The error rate is low, the conditions that cause genuine ambiguity are rare, and sonographers working in the second trimester have a strong track record. The small possibility of a mistake is worth knowing about, not because it should keep you up at night, but because it’s a reminder that ultrasound shows a picture, not a certainty, and a picture depends on the angle, the lighting, and the anatomy doing the posing.