At 13 weeks pregnant, What to Expect Community member DancingBeaner posted an ultrasound picture to her birth month group. “I’m just so curious what those of us who know about skull theory think of baby,” she wrote alongside the black-and-white image of her baby in utero.
She is hoping other moms can help her predict if she is having a boy or a girl based on the curve of the baby’s skull. The guesses quickly followed: “I feel like my son looked like your pic and that my girl’s skulls have been curvier,” replied What to Expect Community mom WowWhatsGood. Others shared their own ultrasound images to help her decide: “Mine is a girl and her head looked like this at 12 weeks,” wrote Kaylieghbae.
Skull theory is popular among parents-to-be — some even say they’ve plugged their ultrasound photos into ChatGPT and asked it to predict the baby’s gender using the skull theory. But can skull shape actually predict fetal sex? Here’s what experts say.
The skull theory claims your baby’s skull shape can predict whether it’s a boy or a girl.
No research or evidence supports the validity of the skull theory.
The best way to know your baby's gender for sure, though, is non-invasive prenatal testing (NIPT) around 9 or 10 weeks.
How does the skull theory claim to predict your baby’s sex?
The skull theory, also known as the skull gender theory, is the belief that you can accurately predict the gender of your baby by simply looking at the fetal skull shape during a 12-week ultrasound.
The theory claims that male skulls are more square and blocky, with a defined brow ridge, angled jaw, and prominent cheekbones. Female skulls, on the other hand, are thought to be more round and oval, with a smaller brow ridge and wider jaw angle.
It’s hard to say where skull theory originated; no record of it exists in traditional medical literature.
Can you really predict gender by skull shape on an ultrasound?
Unfortunately, no. “You cannot tell female or male gender just by looking at the skull of the fetus,” says Lori Lofton, RDMS, RVT, a registered diagnostic medical sonographer at HCA Florida Institute for Women’s Health and Body. “In general, most fetal heads are actually slightly oval,” she says. Given such, if you’re attempting to apply the skull theory, many ultrasounds would sway female, since supporters claim baby girls have a rounder head.
There is also no scientific basis, medical evidence, or peer-reviewed research on the skull theory to support any claims of validity you might see online. It can certainly be fun to speculate and hypothesize, but again, Lofton says it’s not a reliable gender predictor. Any support of accuracy is anecdotal…or a lucky guess.
What are other (more!) accurate gender predictors?
The most accurate way to tell the sex of a baby is by a blood test known as non-invasive prenatal testing (NIPT), says Kyle Graham, M.D., an OB/GYN and medical director at Pediatrix Medical Group. The test looks at DNA from your baby’s placenta in a sample of your blood and can be performed as early as nine weeks into pregnancy, he explains. Results can take up to a week or more to receive, but it’s around 99 percent accurate. “If you want to know the gender of your baby without a doubt, I recommend getting this blood work,” Lofton says.
An abdominal ultrasound halfway through pregnancy, around 17 to 20 weeks (commonly known as an anatomy scan), can also determine fetal sex by using sound waves to take pictures of your baby’s developing anatomy and growth, Lofton says.
“At that time, if the parent wishes to know the baby’s gender, the ultrasound tech will look between the fetus’ legs, and if you see three lines, that indicates female, since those are considered the labias, and if it’s a boy, you’ll see the testicles and a penis which can sometimes look like turtle,” she explains. Anatomy scans are generally very accurate, but if your baby is curled up inside the womb during your ultrasound, it may make the genitals difficult to see.
Now, when it comes to at-home gender prediction kits, Lofton and Dr. Graham both agree there’s a lot less credibility and accuracy. Most gender prediction kits you find in drugstores and online claim to test your baby’s sex hormones using a finger prick or urine sample, but these tests are unfortunately unreliable. Why? Because at-home blood tests are highly prone to error, and urine doesn’t contain any related information (such as DNA or sex hormones) that could predict the sex of a baby.
There’s also a lack of independent, peer-reviewed studies to support DIY gender prediction kits, so they’re not worth counting on, Dr. Graham says. “There are many myths regarding fetal sex, but I’m not aware of any accurate at-home predictors, so I recommend using the vetted and studied NIPT testing or a detailed anatomy scan.”
That being said, if you want to use an at-home gender predictor test or the skull theory for fun, go for it. There’s no harm, but it’s likely still a 50-50 chance you’re having a boy or girl, so Lofton says to “take it with a grain of salt.”
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