What is Skull Theory?

Published on October 19th, 2020 and Updated on April 1st, 2021

When you’re pregnant, the transformations happening in your uterus might always be on your mind. Small hands are forming, a little face is taking shape, and tiny toes are wiggling for the first time. Your unborn baby’s body is developing into something amazing. 

The same goes for your baby’s head.

Your baby’s brain and head start developing soon after conception, but is head development influenced by gender? Can the shape of your baby’s skull give you an indication of whether you should paint the nursery pink or blue?

Skull Theory Gender Prediction 

From reading belly shapes to calendar dates, parents throughout the ages have sought ways to discover their baby’s gender during pregnancy. One gender prediction test theory declares that sugar cravings lead to a little girl. Another theory states that shinier hair and a radiant complexion indicates a little baby boy. 

And then there’s the skull theory. 

What is the skull gender theory? 

The skull theory is a gender prediction method often compared to others such as nub theory or Ramzi theory and are very different from the common gender blood test. It is the idea that male and female babies’ heads develop into different shapes in the womb. Because an abdominal ultrasound or transvaginal ultrasound can show a baby’s head long before it can show a baby’s sex organs, this theory suggests that parents can learn their baby’s gender as early as 12 weeks.

But to understand this theory’s foundations, it may help to understand its most essential tool—the ultrasound.

What an Ultrasound Can Show You, and When 

When a pregnant woman gets an ultrasound, it is used by medical professionals to show images of the baby, the placenta, placenta placement, the amniotic sac, and the ovaries. It’s how doctors take a peek at a baby’s developmental progress and keep an eye out for potential risk factors. 

Most pregnant women will typically have three two ultrasound scans during their pregnancy:

  • Dating Ultrasound – Conducted between week 6 and week 9 of pregnancy, this ultrasound is used to estimate the baby’s due date. Doctors can figure out when to expect your baby’s arrival by examining the ultrasound and determining which developmental stage your baby is in. Medical professionals will also use this ultrasound to:
    • Confirm your pregnancy.
    • Learn whether you’re expecting multiples—twins, triplets, et cetera.
    • Hear the heartbeat (although it will be easier to detect the heartbeat after week 7 of pregnancy).
  • End-of-First-Trimester Ultrasound – Usually scheduled between week 11 and week 14 of pregnancy, this pre-natal ultrasound can provide some exciting and vital information to you and your partner. This ultrasound can help you and your medical provider:
    • Confirm your baby’s due date.
    • Discover if you have any ovarian or uterine anomalies.
    • See your baby’s face, head, and the beginnings of her limbs.
    • Check on the baby’s heartbeat (though you can do this at home any time with the help of our Baby Heart Monitor).
  • Second Trimester Ultrasound – Your doctor will typically schedule your second trimester ultrasound between week 18 and week 22 of your pregnancy. Your baby has done plenty of growing since your first ultrasound. Because of that, your doctor will be able to:
    • Evaluate formation of your baby’s organs and limbs to ensure they’re growing properly.
    • Check the amount of amniotic fluid—the protective fluid surrounding your baby in the womb—to make sure there’s enough.
    • Determine the gender of your baby by viewing the baby’s sex organs.

The second trimester ultrasound can show the sex organs of a baby. But the foundations of a baby’s sex organs begin to develop between week 7 and week 12—long before your second trimester ultrasound.

So why does it take so long for the ultrasound to spot the difference? 

Until babies reach 14 weeks of gestation, both female and male organs are the same size and look to be actually about the same shape. That means you wouldn’t be able to tell the difference between a boy’s sex organ and a girls’ sex organ.

However, after week 14, the male organ begins to develop into its distinct form. By the 18th week, there’s a clear indication of a baby’s gender.

That’s part of the appeal of the skull theory of gender determination—you don’t have to wait 18-22 weeks to take a peek at your baby’s gender. According to this theory, a boy baby’s head shape is different from a girl baby’s head shape, and you can see your baby’s head as early as 11 weeks. Therefore, you should be able to determine the baby’s gender at the end of your first trimester.

How to Use the Skull Theory Gender Prediction Method 

Assuming you’ve heard the saying: a picture is worth a thousand words. That can be especially true for your ultrasound scan images, especially if you’re just looking for one word—boy or girl. 

If you’re trying to use the skull theory to learn your baby’s gender, here are a few key steps to follow as you’re preparing for your end-of-first trimester ultrasound:

1. Get a clear picture

Those who use this gender prediction method agree—the clearer the picture, the better. Since it may be difficult to judge the nuanced differences between a girl’s head shape and a boy’s, high-contrast ultrasounds are your best friend. 

2. Frame the shot so that the baby’s head is in profile

To use the skull theory method, you’ll need to look at a baby’s head in profile. Moms who swear by this method advise the image to be angled so you can see the baby’s skull clearly from front to back. 

 3. Ask for plenty of pictures

If your baby is already a little wiggle worm, it may be hard to get a still shot. Advocates of this method suggest that you ask the technician for multiple images to increase your chances of catching a good one. 

Reading the Results of a Skull Theory Method 

Now that the jelly is off your belly and you’re holding the first images of your child in your hand, it’s time to read between the lines—with some descriptions of skull theory examples to help.

Skull Appearance for a Boy 

This gender prediction method theorizes that baby boys have more square-like jaws and bigger, blockier skulls than girls. Look for a defined brow ridge, angled jaws, prominent cheekbones, and a square chin. 

[Images sourced from Healthline]

Skull Appearance for a Girl 

Smoother, rounder foreheads, round chins, and smaller brow ridges point to a baby girl according to this method:

[Images sourced from Healthline]

Is the Skull Theory Accurate? 

When it comes to skulls and bone shapes, who better to answer this question than Dr. Kristina Killgrove, assistant professor of anthropology at the University of North Carolina, Chapel Hill.

Dr. Killgrove specializes in bioarchaeology—the study of bones found in archeological remains. Armed with decades of education and expertise, Dr. Killgrove has excavated important archaeological burial sites from Ecuador to Italy. 

Needless to say, she knows her bones. 

So here’s what Dr. Killgrove had to say about the skull theory:

“It makes no sense. Anthropologists and archaeologists can, in fact, tell the difference between adult male and female skulls—sometimes. But the skeletons of baby boys and baby girls just don’t look that different.”

In short? The skull theory isn’t exactly—ahem—ahead of the game when it comes to baby gender prediction. 

Possible Origins of the Skull Theory 

Here’s a head scratcher—where did the theory that babies skulls could determine gender even come from? Scientists have believed that the head’s shape held more information than just what hat size would be optimal for an individual for generations. 

Archaeology and Anthropology 

Dr. Killgrove does point out that adult human male and female skulls can—sometimes—be distinguished from another.

  • Male skulls tend to have more prominent brow ridges, squarer chins, angular jaws, and developed ridges at the back of the skull where neck muscles attach to the head. Think of the last cartoon you watched featuring a neanderthal or caveman—those prominent foreheads and heavy jaws are the exaggerated characteristics of today’s human male skull shape.
  • Female skulls have smaller, rounder brow ridges, a more pointed chin, and a smoother, less sloped back area of their skull where the neck muscles attach.

(If your inner bioarchaeologist is awakening—or if you’ve always considered yourself a bit of an Indiana Jones type—you can learn more about the difference between male and female adult skeletons with the help of the Smithsonian National Museum of Natural History).

Bioarchaeologists like Dr. Killgrove use these indicators to guess the gender of a human skull—however, the skull differences between girls and boys don’t fully develop until their pubescent years.

In fact, a baby’s skull stays pretty soft in the womb, and their skull bones don’t fuse and solidify until after delivery. Scientists believe there are two primary reasons for a baby’s soft skull.

1. Making Room for Growth 

A baby’s slightly malleable skull shape gives babies more room to grow in the uterus, which is adjacent to the bladder.

2. An Easier Delivery

An infant’s brain is a bit bigger than their exit door. A soft skull helps a baby emerge into the world from the womb through the birth canal more easily.  

The “soft parts” of a baby’s skull solidify by about 18 months of age, but a human skull isn’t fully formed until the age of 20—which is why bioarchaeologists have an easier time distinguishing male or female skulls in adults rather than infants or children. 

However, the skull isn’t always the most reliable way to determine a skeleton’s gender for adults either. The pelvis, however, is a bioarchaeologist’s gender determining go-to. A human female’s pelvis is broader and more bowl-shaped than a male’s—which tends to be more square. Why? The unique shape of a woman’s pelvis allows her to carry a child.

Men’s pelvises? Not so much.


Run your fingers over your head—you probably feel bumps and divots that have been a part of you for your whole life. But in the 1700s, a German doctor named Franz Joseph Gall suspected those bumps could speak volumes about a person’s head, health, and heart. He called this pseudoscience “phrenology.” Dr. Gall claimed that those little bumps on your head could indicate a variety of characteristics, including: 

  • Reproductive abilities
  • Social skills
  • Tendency to commit crimes like theft and murder
  • Proclivity towards certain biblical sins
  • Musical ability
  • Kindness
  • Intelligence
  • Ability to be educated
  • Sense of colors
  • Math skills
  • Religious strength
  • Poetic ability

Maybe phrenology was the grandfather of the Skull Theory gender prediction method. However, no grounded scientific studies have verified Dr. Gall’s claims—or the baby skull theory. So reading someone’s skull-bumps, or a baby’s head in an ultrasound, is no more telling than a fortune teller reading your palm. 

Instead of waiting until the end of your first trimester to stare at the ultrasound to glean your baby’s sex, you can find out the gender of your baby as early as 8 weeks into your pregnancy.

You just need a little help from SneakPeek.

Getting Ahead of Your Baby’s Gender with SneakPeek 

 While your baby grows from head to toe, your head might be filled with questions about your future child. Get answers sooner than ever with The SneakPeek Early Gender DNA Test. 

This simple, safe test can be taken as early as 8 weeks into your pregnancy—weeks before your first ultrasound. Just order the test kit, complete the test in the comfort of your home, and send it to SneakPeek Labs in the provided pre-paid packaging. You’ll learn your baby’s gender soon after your kit is received. 

What’s even better than finding answers early? Being confident in those answers. The SneakPeek Early Gender DNA Test has proven to be 99.9% accurate in laboratory studies. 

Learn your baby’s gender sooner than ever with SneakPeek!



VeryWell Mind. Phrenology’s History and Influence. 


Healthline. Can Using Skull Theory Reveal If You’re Having a Boy or a Girl?. https://www.healthline.com/health/pregnancy/skull-theory#what-skull-theory-is

What to Expect. Fetal Development: Your Baby’s Sex. https://www.whattoexpect.com/pregnancy/fetal-development/fetal-sex-organs-reproductive-system/

What to Expect. Level 2 Ultrasound: Is It Only Scheduled When You’re an “Older Mom”? https://www.whattoexpect.com/pregnancy/ask-heidi/level-2-ultrasound.aspx

WebMD. Prenatal Ultrasound. https://www.webmd.com/baby/ultrasound

Smithsonian Natural History. Is The Skeleton Male or Female? https://naturalhistory.si.edu/sites/default/files/media/file/wibskeletonmaleorfemalefinal.pdf

Healthline Parenthood. How Early Can You Hear Baby’s Heartbeat on Ultrasound and By Ear? https://www.healthline.com/health/pregnancy/when-can-you-hear-babys-heartbeat

SneakPeek aims to provide the most accurate and up-to-date information to help our readers make informed decisions regarding their health before, during, and after pregnancy. This article was written based upon trusted scientific research studies and/or articles. Credible information sources for this article are cited and hyperlinked.

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