Skull theory: can head shape tell boy or girl?

What skull theory claims

Skull theory says a scan picture from about 12 weeks, counted from your last period, shows boy or girl in the shape of the head. It needs a clear side view, the profile, “where the skull can be measured from front to back,” as Healthline puts it (Healthline, 2020). Healthline’s medically reviewed summary lists the claims:

Nobody seems to know who started it. “It’s hard to say how skull theory got started,” Live Science wrote (Live Science, 2014). We describe the claims so you know what people mean, not as a checklist. None of them is a reliable sign on a scan picture, for the reasons below.

Where the idea comes from

The claims echo the way forensic anthropologists estimate sex from adult skeletons. The ideas “appear to be drawn from anthropological studies of adult skeletons,” Live Science noted. An anthropology textbook from the American Anthropological Association lists skull features that tend to differ between adult females and males (Organ and Byram, 2023):

• The brow. The glabella, the spot between the eyebrows, “tends to be flat” in females and is “more rounded and protruding” in males.
• The edge above each eye socket. It “tends to be thin and sharp in females and blunt and thick in males.”
• The bump behind the ear. Males tend to have “longer and wider mastoid processes.”
• The back of the skull. The nuchal lines there “are usually more pronounced in males.”
• The chin and jaw. Males tend to have a more prominent chin, and a jaw angle “closer to a right angle.”

Skull theory’s brow ridge, square chin and angled jaw echo that list.

How sure experts are, even with adult skulls

With a whole adult skull in hand, experts still don’t always get it right. In a 2008 study, an anthropologist at the University of California, Santa Barbara, tested by-eye scores for five of the features above on 304 skulls of known sex. They came from people of European American, African American and English ancestry. The best statistical model sorted 88% correctly (Walker, 2008).

The skull isn’t the first choice, either. Guidance from SWGANTH, a scientific working group for forensic anthropology, says methods based on the pelvis “are preferred” (SWGANTH, 2010).

When the differences appear

These differences take shape mainly at or after puberty. An open forensic anthropology textbook explains it (Bailey and Harvey, 2023). “Prior to the development of secondary sexual characteristics, however, there is limited sexual dimorphism in skeletal features, rendering macroscopic methods unreliable before puberty,” it says. Macroscopic means by eye, and sexual dimorphism means differences between the sexes.

Until you get to maturity or at least puberty you just don’t get these sexually dimorphic features of the skulls in males and females.

Kristina Killgrove, bioarchaeologist, University of West Florida, speaking to Live Science in 2014

Researchers measuring children’s scans have found small average differences much earlier. A 2023 study measured 22 midface dimensions on CT scans of 431 children of East Slavonic descent, from birth to age 5. At birth, the differences between boys and girls were low for most of them, then grew during the first year (Syutkina and colleagues, 2023). Even so, SWGANTH’s guidance says it’s “generally unadvisable to assess sex for fetal/infant/child (under 12 years) remains because valid sex assessment techniques are unavailable.” That’s with real bones to examine, not a picture.

What a 12-week scan picture can and can’t show

Skull theory works from pictures taken at about 12 weeks, such as those from the medical scan at 11 to 14 weeks. Hospitals in England offer one at 11 to 14 weeks (NHS), and Can you find out the gender at 12 weeks? explains the U.S. version. Here’s what that picture is up against:

• The bones are still forming. At 12 weeks, the NHS says, “The skeleton is made up from tissue and hardening into bone” (NHS). The international guideline for this scan expects skull bone to be visible “by 11 completed gestational weeks,” only a week or so earlier (ISUOG, 2023).
• The baby is small. About 5.4 centimeters from head to bottom at 12 weeks, “about the size of a plum,” the NHS says. Its head is about 2 centimeters wide, as the Milan study below found.
• The picture is zoomed in. To measure the baby’s length, the guideline says, “The image should be magnified to fill most of the width of the ultrasound screen.” So the size of a head on a print says more about the zoom than about the baby.
• It’s one view, at one moment. The profile is a particular cut down the middle of the baby, the midsagittal view. Before measuring, the guideline wants the baby in a neutral position, “neither flexed nor hyperextended.” A print freezes one angle at one moment, and nub theory explains how that can mislead.
• The sex is read somewhere else. When the sex is looked at during this scan, the guideline says the evaluation “is based upon the orientation of the genital tubercle.” That’s a small bud between the legs, not the skull.

At the medical scan, this view also lets the team assess the face, including the forehead, nose and jaw, the guideline says. Only your doctor or midwife can say what yours showed.

Are boys’ and girls’ heads different before birth?

In size, a little, on average, and more clearly later in pregnancy than at 12 weeks. These three studies measured head size on ultrasound scans. None looked at the brow, chin or jaw:

• Milan, Italy, at 11 to 14 weeks. The analysis included all 1,052 healthy women with single pregnancies enrolled at a children’s hospital from 2020 to 2023, scanned at 11 weeks to 13 weeks 6 days. The middle value for head width was 20.7 millimeters for 523 boys and 20.4 for 529 girls, a gap that wasn’t statistically significant. The authors wrote that “fetal biometry were comparable” (Parisi and colleagues, 2025).
• Rotterdam, the Netherlands, through pregnancy. Of 9,778 mothers in the Generation R study, the analysis included 8,556 single babies born alive from 2002 to 2006. In the first trimester (median 12.5 weeks; 1,782 analyzed), boys were longer from head to bottom by 0.12 standard deviations. From the second trimester, their head circumference was larger by 0.30 standard deviations (Broere-Brown and colleagues, 2016). A standard deviation measures how much babies normally vary, so even the larger gap is under a third of the usual spread.
• Leuven, Belgium, from 20 weeks. At the University Hospital Leuven, researchers used 27,680 scans of 12,368 low-risk single pregnancies, all of Caucasian ethnicity, from 2002 to 2012, and compared boys and girls from 20 weeks. From then on, boys’ head measurements were significantly larger, “equating to a 3-day difference at 20–24 weeks.” At 24 weeks, the middle head width was 60.4 millimeters for boys and 58.9 for girls (Galjaard and colleagues, 2019).

Why can’t an average sort one baby?

An average describes a group. In the Leuven data, the boys’ whole range of head widths sat only a little above the girls’. The boys’ 5th percentile lined up with the girls’ 10th, and their 90th with the girls’ 95th. So plenty of girls have wider heads than plenty of boys. A 1.5-millimeter gap between the middle values at 24 weeks can’t tell you which group one baby belongs to. At 11 to 14 weeks, the Milan study found no clear gap in head width.

These are group figures, not a yardstick for your baby: questions about your baby’s growth go to your doctor or midwife.

Has anyone tested skull theory?

Not that we could find. In October 2026, we searched PubMed, Crossref, the Directory of Open Access Journals and the wider web for a study that tested skull theory on scan pictures. We found none. The studies of how sex is read at early scans look at the genital tubercle, not the skull.

Others have said much the same. Healthline’s medically reviewed article said in 2020 that “there is little to no evidence from peer-reviewed journals.” Asked about skull theory by Live Science in 2014, the bioarchaeologist Kristina Killgrove said: “It makes no sense.”

Why it can seem to work

Accuracy figures do circulate. Healthline reported in 2020 that even supporters put its accuracy “only between 70 and 95 percent.” We found no published study behind any such figure.

Any guess also gets a head start. In the U.S., about 105 boys were born for every 100 girls from 1940 to 2002 (CDC). So a guess, even a coin toss, is right about half the time, and a skull-theory call that turned out right proves nothing on its own. Guessing with friends is fun; just save the paint and the reveal for a confident answer.

Skull theory has company. Ramzi theory goes by the placenta’s position, the heart-rate myth by how fast the heart beats, and the Chinese gender chart by the mother’s age and the month of conception; each has its own article. Our boy or girl quiz puts seven old wives’ tales to the test, just for fun.

What works instead

A scan or a blood test detects the baby’s sex, from anatomy or chromosomes. These are the routes that work, though none is 100% sure:

1. From about 10 weeks: a blood test. Cell-free DNA screening (NIPT) is a test your doctor or midwife orders to screen for chromosomal conditions, and it can report the sex as a side result. It “can be done as early as 10 weeks of pregnancy,” Cleveland Clinic says, and the sex result is “highly accurate, but not 100% accurate” (Cleveland Clinic).
2. From about 12 weeks: a scan that reads the genitals. Early on, the call rests on the genital tubercle. In a 1999 London study of 524 women scanned at 11 to 14 weeks, more babies could be called each week, but the accuracy of the calls made didn’t change with the week (Whitlow and colleagues, 1999).
3. About 18 to 22 weeks in the U.S.: the anatomy scan. This medical scan checks how your baby’s organs and body are developing. “In most cases, you can also learn the sex,” Cleveland Clinic says, depending on how your baby is lying (Cleveland Clinic). In England, the scan is at 18 to 21 weeks, finding out “is not offered as part of the national screening programme,” and some hospitals don’t tell (NHS).
4. At birth. No test needed.

When can you find out your baby’s gender? puts every option on one timeline. How accurate is a gender ultrasound at 13, 14, 15 and 16 weeks? has the study numbers week by week.

Past your 12-week picture, or waiting for it? Add your due date to the planner to see which of our sessions are ready for you now and which open next. The date you enter stays on your device.

Finding out in Glendale, from 13 weeks

Skull theory guesses from a still picture. At our studio in Glendale, Los Angeles County, if we can’t tell with confidence, we don’t guess (our promise). When we can’t be sure, we say so, and you come back for a free re-check. No ultrasound can promise 100%.

Our Gender Confirmation is a 2D scan, “the classic black-and-white picture,” with 8 black-and-white prints. We book from 13 weeks, and we’re surest from 15. At 12 weeks? Sometimes, but rarely with confidence, so we book from 13 weeks. If you can wait until 15, do: we’re more confident, and a second look is less likely. Once we have a clear view it can be over in 2 minutes. If your baby is being shy, we take longer.

It’s a keepsake, not a medical exam: it doesn’t check your baby’s growth or health, 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 has their statements. Find out from 13 weeks, confirm what you’ve been told, or see prices.

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