Yes, and wrong calls are far more common early on. The NHS says a sonographer can’t be 100% certain of a baby’s sex. In a 2014 study at one Australian practice (640 babies across pregnancy), 75% of babies scanned from the side at 11 to 14 weeks got a correct call, counting those left without one. At the 18-to-21-week scan in two Danish hospitals, sex was called for 86.7% of 5,786 babies, and 8 in 1,000 calls were wrong.
This article walks through why calls go wrong, how a careful sonographer guards against it, and what to do when two results disagree.
How often is a gender ultrasound wrong?
A scan reads the baby’s anatomy, so what it shows is the baby’s sex. How often it’s wrong depends on the week.
In the Australian study, seven sonographers at a specialist private practice in the Illawarra, New South Wales, scanned 640 babies across all three trimesters. Each call was checked against the baby’s sex after birth (Kearin and colleagues, Australasian Journal of Ultrasound in Medicine, 2014). From 11 to 14 weeks, 75% of babies got a correct call; counting only the calls made, 91% were right. The abstract doesn’t say how many babies were in each week group.
Early on, boys and girls both have a small bud between the legs, the genital tubercle, and its length doesn’t tell them apart. A 1990 study of internal scans at 13 to 16 weeks, at Rambam Medical Center in Haifa, Israel, called the length “not diagnostic” at that stage (Bronshtein and colleagues, Journal of Clinical Ultrasound, 1990). In the late first and early second trimester, the call rests mainly on which way the bud points. It points up in a boy and down in a girl, a 2009 review by Israeli doctors explains (Odeh and colleagues, Obstetrical & Gynecological Survey, 2009).
A boy’s bud tilts up more as he grows. A London fetal medicine center scanned 172 babies at 11 to 14 weeks, just before a chromosome test (chorionic villus sampling), and called a boy when the angle was over 30 degrees, a girl otherwise. At 11 weeks, 56% of boys were called girls. At 12 weeks it was 3%, and at 13 weeks none (Efrat and colleagues, Ultrasound in Obstetrics & Gynecology, 1999). The Australasian Sonographers Association says sex calls by ultrasound before 12 weeks “should be discouraged” (ASA, 2022).
The cord between the legs
A loop of umbilical cord can look like a penis. A radiologist in Abbottabad, Pakistan, noted this in a 1996 report: the penis “can be confused with a loop of cord” (Idrees, Journal of Ayub Medical College Abbottabad, 1996). It’s a general caution, not a finding: the study itself, of 300 women at 26 to 30 weeks, put none of its wrong calls down to the cord.
Parts that can pass for the other sex
Early on, a girl can pass for a boy. The Haifa team warned that judging 13-to-16-week scans by the signs used later in pregnancy “is hazardous, especially for determining the male sex.” Later, folds can still mislead. A scrotum without testes “can be confused with labia majora,” the outer lips, the Pakistani report notes.
A hidden view
The NHS says that “if your baby is lying in an awkward position, it may be difficult or impossible to tell” (NHS). At United Christian Hospital in Hong Kong, a 2016 study scanned 400 pregnancies at 11 to 13+6 weeks, judging from a side view. A baby lying face down significantly cut the chance of making a call (Kong and colleagues, Journal of Medical Ultrasound, 2016). The Pakistani report found position a problem “especially so when foetal spine is anterior,” with the baby’s back toward the probe, and in breech babies.
Seeing nothing isn’t proof of a girl, because a scrotum can be out of view. At a private clinic in Addis Ababa, Ethiopia, 435 women were scanned from 16 weeks to term, and 269 mothers of 275 babies were reached after birth. When nothing showed between the legs, the call was “probably female.” Only 17 of those 30 babies were girls (Gelaw and Bisrat, Ethiopian Journal of Health Development, 2011).
A call from one view
Early calls rest on one side view. In the Australian study, first-trimester calls used “the sagittal plane only,” a lengthwise view, while later calls used that or a cross-section.
Who scans matters too. Success rates differed between the seven Australian sonographers.
Body build and the scan conditions
The Ethiopian team says accuracy depends on the sonographer’s skill, the baby’s position, the amount of fluid around the baby and the “thickness of abdomen.” Low fluid (oligohydramnios) is “a recognized cause of failure to establish foetal sex,” the Pakistani report adds.
Twins: which baby is which
With twins, each baby needs its own clear view, and the two must be kept straight. The U.S. practice guideline for standard pregnancy scans asks for twins’ genitalia to be documented “when visualized” (ACR–ACOG–AIUM–SMFM–SRU). One option in international guidance is labeling twins by where they lie, right and left or lower and upper, written down so later scans stay consistent (ISUOG, 2025).
Even so, the twins labeled A and B “may not necessarily be delivered in that order,” ISUOG notes. A boy-girl pair can arrive the other way round without any call being wrong. Cell-free DNA blood tests are “not as accurate if you are pregnant with more than one baby,” MedlinePlus says (MedlinePlus). Can you see twins at 6 weeks? covers how twins are counted.
Which way do mistakes go?
“It’s a girl,” then a boy
In a London study, this was the more common slip in the earliest weeks: at 11 weeks, 56% of boys were called girls, against 5% of girls called boys. In Australia, boys under 13 weeks were more likely to be called wrong or left without a call. Later, a hidden scrotum can make a boy look like a girl.
“It’s a boy,” then a girl
At 13 to 16 weeks, the Haifa team warned that a boy call was the riskier one if based on later-pregnancy signs. A loop of cord can look like a penis.
Where studies disagree
In Hong Kong, at 11 to 13+6 weeks, boys were called right more often than girls, and in Australia the same held after 13 weeks. At 18 to 21 weeks in Denmark, a 2020 correction found “no difference” between girls and boys.
How a careful sonographer makes the call
These habits make a call as sure as it can be. None makes it certain.
Waits for the right week. The 2009 review says no call should be attempted before 12 weeks, because that early “it is relatively inaccurate.”
Gets a true side view. Early on, the call needs a lengthwise view through the middle of the baby, to judge the bud’s angle against a line along the lower back.
Adds views later. By the late second trimester, sonographers can look at the genitals directly. After the Danish study, Denmark’s fetal medicine society wrote a national guideline: sex checks “including sagittal, transverse and tangential projections” are now recommended at the anomaly scan there.
Waits for a clear view, or asks you back. At the 20-week scan in England, the scan “can sometimes take longer” if the baby isn’t lying right for clear pictures. You may be asked to go for a walk or eat something and come back “so all the checks can be done,” though finding out the sex isn’t one of them (NHS).
Tells you how sure they are. The Australasian Sonographers Association says sonographers “should state that errors exist with early predictions,” and explain that the sex can be assessed “with more accuracy” later.
If two results disagree
Two scans
In the studies above, early calls were the least reliable, so a later, clearer look deserves more weight. If you’re unsure which to believe, get another look at 15 weeks or more, or ask at your anatomy scan. In the Haifa study, a call was possible in 76% to 80% of pregnancies at 13 to 14 weeks, and in 88% to 96.7% at 15 to 16 weeks.
In the U.S., the guideline for standard scans says the baby’s anatomy “may be adequately assessed by ultrasound after approximately 18 weeks.” It lists the external genitalia “If medically indicated or the patient wants to know,” so say that you want to know. In England, finding out the sex “is not offered as part of the national screening programme.” You may be able to find out at the 20-week scan, usually done at 18 to 21 weeks, “but this depends on the policy of your hospital.”
A scan and a blood test
Cell-free DNA screening, also called NIPT, is a blood test your doctor or midwife can order from as early as the 10th week. It checks whether the baby is more likely to have certain chromosome conditions, and it can also be used to determine the sex. It can’t tell you “for sure” whether a baby has a chromosome disorder (MedlinePlus). When can you find out your baby’s gender? covers every way to find out.
If its result and a scan disagree, tell your doctor or midwife. There are several possible explanations, a few of them medical, and only they can look into it. They may suggest a genetic counselor. A keepsake scan can’t settle it.
A 2020 study, led by a researcher at the U.S. testing company Natera, gives one laboratory’s figures. Among its 1,301,117 cell-free DNA tests of one type (SNP-based) that reported normal sex chromosomes, a mismatch with a scan or other clinical findings was reported to the lab in 91. That’s about 1 in 14,300 (Dhamankar and colleagues, Obstetrics & Gynecology, 2020).
Of the 83 with follow-up, 28 came down to human error, mostly a labeling mistake when blood was drawn or a scan that called the sex wrong. In 30, chromosome testing and clinical details showed a difference in sex development. These are rare conditions involving genes, hormones and reproductive organs, and only medical specialists can assess them (NHS). In 25, no diagnosis was possible. The study counted only mismatches someone reported, so it can’t say how often results differ overall, or what’s behind any one result. Your doctor or midwife can look into yours.
Our Gender Confirmation is a 2D scan, “the classic black-and-white picture,” at our studio in Glendale, Los Angeles County. We book from 13 weeks, and we’re surest from 15. If you can wait until 15, do: we’re more confident, and a second look is less likely. Twins or triplets? Come at 14 to 15 weeks, and tell us when you book.
Once we have a clear view it can be over in 2 minutes. If your baby is being shy, we take longer. When we can’t be sure, we say so, and you come back for a free re-check: if we can’t tell with confidence, we don’t guess (our promise). No ultrasound can promise 100%.
If what we see is different from what you’ve been told, we tell you what we see, and how sure we are. What a test result means is a question for your doctor or midwife. A keepsake scan doesn’t check your baby’s anatomy, and it doesn’t replace the anatomy scan or any appointment with your doctor or midwife. The FDA and the AIUM discourage scans done only for keepsakes (FDA, AIUM); Safety, in plain words quotes them. See what a Gender Confirmation includes and our prices.
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Questions
Can a gender ultrasound be wrong?
Yes, and early calls are the least reliable. The NHS says a sonographer can’t be 100% certain of a baby’s sex. At one Australian practice in 2014, 75% of babies scanned from the side at 11 to 14 weeks got a correct call, counting those with no call. Even at the 18-to-21-week scan in two Danish hospitals, 8 in 1,000 calls were wrong.
Can a 12-week gender scan be wrong?
Yes. At 12 weeks, the call rests mainly on which way a small bud between the legs points, seen from the side. In a 1999 study at a London fetal medicine center, 172 babies were scanned at 11 to 14 weeks, and 98.7% of calls at 12 weeks were right, against 70.3% at 11. The Australasian Sonographers Association says calls before 12 weeks should be discouraged.
Can an ultrasound say girl and it turns out to be a boy?
Yes, especially early. In a London study of 172 babies scanned at 11 to 14 weeks, 56% of boys at 11 weeks were called girls, because a boy’s genital tubercle tilts up as he grows. Later on, a scrotum can be out of view, so seeing nothing between the legs isn’t proof of a girl.
Can an ultrasound say boy and it turns out to be a girl?
Yes. At 13 to 16 weeks, the length of the bud between the legs doesn’t tell girls from boys, and a 1990 study called a boy call based on later-pregnancy signs “hazardous.” A loop of umbilical cord can also look like a penis. At 18 to 21 weeks, a Danish study found no difference in accuracy between girls and boys.
What if my NIPT and ultrasound gender don’t match?
Tell your doctor or midwife. There are several possible explanations, a few of them medical, and only they can look into it; they may suggest a genetic counselor. In a 2020 study of one U.S. lab’s 1,301,117 cell-free DNA tests, a mismatch was reported to the lab in 91, about 1 in 14,300. A keepsake scan can’t settle it.
Two gender scans disagree. Which should I believe?
Neither is 100% sure, but early calls are the least reliable, so a later, clearer look deserves more weight. Get another look at 15 weeks or more, or ask at your anatomy scan. In the U.S., the guideline for that scan lists the genitals if medically indicated or if you want to know. The FDA and the AIUM discourage scans done only for keepsakes.
“90% sure” is a sonographer’s judgment of how clear the view was, not a statistic. What raises or lowers it, what to ask, and when to buy pink or blue.
A blood test your doctor or midwife orders can report the sex from about 10 weeks, and ultrasound calls get surer as the weeks pass. Every way, week by week.
No scan is 100% sure. In a London study of 524 women, 79% of all babies scanned at 13 weeks got a correct call, and 90% at 14. What each study found, by week.
Updated October 13, 2026
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