How accurate is a gender ultrasound at 13, 14, 15 and 16 weeks?

Why accuracy climbs week by week

What a scan shows is the baby’s sex, read from the outside genitals, and at 11 weeks those are still forming. Counting from the first day of your last period, Mayo Clinic says that by the end of week 11, “the outer genitals start developing” (Mayo Clinic).

Early on, the sonographer reads a small bump between the legs, the genital tubercle. In the late first and early second trimester, a 2009 review explains, sexing is based mainly on the tubercle’s direction, the “sagittal sign”: pointing down suggests a girl, and pointing up, a boy. Later in the second trimester, it’s based on a direct view of the genitals (Odeh and colleagues, 2009).

That direction gets easier to read. In a Tunisian study, the tubercle’s angle grew significantly with the baby’s length in boys, though not in girls (Chelli and colleagues, 2009). A Rotterdam team writes that accuracy is low until about 13 weeks, after which the difference between boys’ and girls’ angles gets easier to see (Bogers and colleagues, 2018).

The angle, at 11 to 14 weeks

The main early method measures an angle in the midsagittal view, a side-on view down the middle of the baby. The Tunisian team measured the tubercle’s angle “to a horizontal line through the lumbosacral skin,” the skin over the lower back. They and an Israeli team used the same rules (Efrat and colleagues, 2006):

• More than 30 degrees: a boy.
• Under 10 degrees, parallel to the line or angled toward it: a girl.
• In between: no call. Earlier studies classed these angles as indeterminate, a 2024 Saudi study notes (Alfuraih and colleagues, 2024).

Later, a direct look

As the weeks go on, sonographers can look at the genitals themselves. The 2009 review lists landmarks such as the scrotum, the midline raphe (seam) of the penis and the labial lines. A Scottish NHS maternity guideline for the 20-week medical anomaly scan says a boy “should be determined by the clear visualisation of the penis and scrotum” (NHS Greater Glasgow and Clyde, 2022).

What the studies found at 11 to 14 weeks

These studies report results by week or by the baby’s size: the crown-rump length (head to bottom) or the biparietal diameter (head width). Read each figure with its “out of how many”: some count only babies who got a call, others count every baby scanned.

• London, 1999: 524 women at 11 to 14 weeks. An unselected group at a university obstetrics department, scanned over the belly, and internally for 26% when needed, in side and cross-section views. The sex was checked at birth. A call was possible for 59% of babies at 11 weeks, 87% at 12, 92% at 13 and 98% at 14. Counting every baby scanned, the call was right for 46%, 75%, 79% and 90%. When a call was made, its accuracy didn’t change with the week. The abstract doesn’t say how many babies were scanned at each week (Whitlow and colleagues).
• Israel, 2006: 656 pregnancies at 12 to 14 weeks. Belly scans with the angle method, at a hospital in Petah Tikva. A call was possible for 613 babies (93%), and the sex was confirmed in 555 newborns. Among the calls, “boy” was 99% to 100% right at every week. “Girl” was right 91.5% of the time at 12 weeks to 12 weeks 3 days, 99% at 12+4 to 12+6, and 100% at 13 to 13+6 (Efrat and colleagues).
• Tunisia, 2009: 312 babies at 11 to 14 weeks. A prospective study in Tunis, with belly scans and the angle method. A call was possible for 89.7%, and the abstract gives accuracy as 85.7% “of fetuses,” without saying whether babies with no call count. Accuracy rose significantly over the weeks in boys, but not in girls (Chelli and colleagues).
• Spain, 2016: 672 babies at 11 weeks to 13 weeks 6 days. A prospective study at a university hospital in Granada, with belly scans and the angle method. A call was possible for 608 (90.5%), and 532 of those 608 calls (87.5%) were right (Manzanares and colleagues).
• Italy, 2004: 2,593 babies, by head width. Internal or belly scans, or both, looked for the sagittal sign in babies with head widths of 18 to 29 millimeters. Results were checked against the sex at birth or a chromosome test. A call was possible for 2,374 (91%), and the sex was known for 2,188. From 22 millimeters, calls were 99% to 100% right; from 24 millimeters, 100% in babies without genital anomalies (Mazza and colleagues, 2004).
• Italy, 2001: 32 IVF babies, each scanned three times. With IVF, the day of fertilization is known. At the first look, up to 69 days after fertilization (11 weeks 6 days, counted from the last period), 29 babies got a call. Of those, 22 calls were right: 6 of 13 for boys and 16 of 16 for girls, the abstract says. At the second and third looks, 70 to 79 days after fertilization, every call was right (Mazza and colleagues, 2001).

At 15 and 16 weeks, and later in pregnancy

None of the studies above reports 15 or 16 weeks on its own. The closest evidence sits on either side:

• After 13 weeks. The 2009 review concluded: “After 13 weeks, it is accurate in 99% to 100% of cases without malformed external genitalia.” Its abstract doesn’t say how many babies that covers, or whether babies with no call count (Odeh and colleagues).
• At 18 to 21 weeks. A Danish team checked the sex noted at medical anomaly scans, from 18 weeks to 21 weeks 6 days, against the national birth register. Across 5,786 single pregnancies at two fetal medicine departments, with births in 2011, the sex was determined at 86.7% of the scans, and 99.2% of those calls were right. That “still leaves 8/1,000” babies with the wrong sex, the authors wrote. A 2020 correction found that accuracy didn’t differ between girls and boys (Størup and colleagues, 2019).

Even at 20 weeks, the Scottish NHS guideline warns that “it is not always possible to determine the sex of the baby.”

Why a careful sonographer won’t call it too early

The Australasian Sonographers Association’s 2022 position statement says: “Sex determination by ultrasound prior to 12 weeks should be discouraged.” Sonographers “are under no obligation” to give the sex at a first-trimester scan for social reasons, it adds (ASA, 2022). The 2009 review says an attempt “should not be made before 12-weeks’ gestation,” because “this early, it is relatively inaccurate.” The large Italian study drew its line by size: no call below a head width of 22 millimeters, which the Rotterdam team equates to 12 weeks 5 days.

So “not sure yet” is part of the result, not a failure. In London, 41 of every 100 babies at 11 weeks got no call, against 2 in 100 at 14 weeks. Can an ultrasound be wrong about the gender? explains how calls go wrong, and “90% sure it’s a girl” explains what a sonographer’s confidence means.

What lowers accuracy

• An early week. In a 2016 study of 400 pregnancies at a Hong Kong hospital, at 11 weeks to 13 weeks 6 days, accuracy before 12 weeks was significantly lower than after (Kong and colleagues, 2016). In Spain, accuracy rose with the baby’s length and the week.
• The baby’s position. In the Hong Kong study, a baby lying prone (face down) made a call significantly less likely.
• Body build. In Spain, calls were less accurate as the mother’s body mass index (BMI) rose. In the Hong Kong study, maternal obesity made a call less likely.
• Experience. In the Hong Kong study, two newly trained sonographers reached 71.4% to 78% accuracy after 200 scans; the abstract doesn’t say how babies with no call were counted. The authors concluded it takes about 200 scans to pass 70%. In London, by contrast, results didn’t differ between operators.
• The method and equipment. The Rotterdam team read 426 stored 3D scans from 112 pregnancies, at 9 to 13 weeks, in a virtual-reality system. It could measure the angle in only 235, partly because the scans weren’t aimed at the genitals. Only 54.9% of those measurements predicted the sex correctly.
• Genitals that are hard to read for a medical reason. The 2009 review notes that a wrong call “may occur when the external genitalia are malformed.” Only a medical specialist can assess that, through your doctor or midwife; a keepsake scan doesn’t check the baby’s anatomy.

Why the numbers online clash

Figures quoted online differ for reasons like these:

• Different weeks. A figure from 11 weeks and one from 14 can both be right, as the London results show.
• Different “out of how many.” In Spain, 87.5% of the calls made were right, but counting the 64 babies with no call, 532 of 672 (79%) got a correct one. Leaving out the babies with no call makes a figure look higher.
• Boy calls versus girl calls. In Israel, “boy” calls were right at least 99% of the time at every week, but “girl” calls only 91.5% early in week 12. One overall figure hides a gap like that.
• Different methods and pictures. The Saudi team, which measured angles on stored images, warned that this could overstate accuracy, as their quality “may not reflect the variability encountered in a real-world clinical setting.”
• Study conditions or everyday care. The Danish figures come from regular clinical anomaly scans, where sex determination was “not mandatory, as there was no national guideline.” The authors wrote that it “should be standardised” (Størup and colleagues).

Choosing your week

The studies point one way: later scans give more calls and, in most of them, fewer wrong ones. As a rough guide, counted from the first day of your last period:

• 11 to 12 weeks: more babies get no call, and more calls are wrong. The ASA discourages calls before 12 weeks.
• 13 to 14 weeks: in the studies, most babies could be called and most calls were right, but a second look may be needed.
• 15 to 16 weeks and later: past the 13-week mark where the 2009 review’s 99% to 100% begins, for babies without genital malformations, though no scan is 100% sure.

At any week, a baby lying the wrong way can still mean “come back later.” Blood tests are a separate route: When can you find out your baby’s gender? sets out every option, week by week.

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Why we book from 13 weeks, and wait for a clear view

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. At 12 weeks? Sometimes, but rarely with confidence, so we book from 13 weeks. 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. As our promise puts it: “If we can’t tell with confidence, we don’t guess. You come back, and the second look costs nothing.” No ultrasound can promise 100%.

Irina, our lead sonographer, has nearly two decades of experience and more than 30,000 pregnancy ultrasounds, and we’ve done more than 20,000 gender sessions. From 16 weeks, you can also find out during a 4D Bonding session, if you’d like to know (prices). It’s a keepsake, not a medical exam: it doesn’t check your baby’s anatomy or health, or 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 has their statements, and Is an early ultrasound safe for the baby? sums up the evidence. Find out from 13 weeks, or confirm what you’ve been told.

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