Key takeaways

• Skull theory is not scientifically proven, and skull shape on an ultrasound cannot reliably predict the sex of your baby.
• Medical methods like ultrasound scans and DNA-based sex prediction tests are far more accurate than prediction theories.
• Some tests can predict your baby’s sex just 6 weeks into your pregnancy with over 99% accuracy.

What is the skull gender theory?

Parents throughout the ages have sought ways to discover their baby’s sex during pregnancy. One old wives’ tale for gender prediction declares that sugar cravings lead to a little girl. Another theory states that shinier hair and a radiant complexion indicate a little baby boy. The skull theory is a sex prediction method often compared to others, such as nub theory or Ramzi theory, and are very different from the common early gender 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 sex as early as the 12 week ultrasound.

The history of skull theory

Where did the theory that a baby’s skull could determine sex even come from? Scientists have long believed that the head’s shape holds a variety of important information.

For example, bioarchaeologist Dr. Killgrove does point out that adult human male and female skulls can sometimes be distinguished from one 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. Prominent foreheads and heavy jaws are the exaggerated characteristics of today’s human male skull shape.
• Smaller, rounder brow ridges, a more pointed chin, and a smoother, less sloped back area of the skull where the neck muscles attach points to a female skull.

(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 sex of a human skull. However, the skull differences between girls and boys don’t fully develop until their pubescent years.

In addition, in the 1700s, a German doctor named Franz Joseph Gall suspected that the bumps and divots on a person’s head could determine aspects of a person’s health and heart. He called this pseudoscience “phrenology.” However, no grounded scientific studies have verified Dr. Gall’s claims or the baby skull theory. To understand this theory’s foundations further, it may help to understand its most essential tool—the ultrasound.

Skull theory and ultrasounds: How does the skull theory work? 

When a pregnant individual 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 people will typically have two to three ultrasounds unless there are any concerns.

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 sex determination—you don’t have to wait 18-22 weeks to take a peek at your baby’s sex. 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 sex at the end of your first trimester.

How to use the skull theory gender prediction method

If you’re trying to use the skull theory to learn the sex of your unborn baby, 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 sex 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. Parents 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

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 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 sex 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.

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 ahead of the game when it comes to baby sex prediction.

More reliable ways to learn your baby’s sex

While methods like skull theory can be fun for early sex prediction, many expectant parents want answers they can trust. If you’re hoping to predict the sex of your baby with greater confidence, there are options that offer significantly higher accuracy.

Early gender tests

For those seeking an early prediction, non-invasive options like DNA-based blood tests offer a highly accurate alternative. Tests such as the SneakPeek® Early Gender Test and other types of NIPT (Noninvasive Prenatal Testing) analyze fetal DNA present in the bloodstream to predict your baby’s sex as early as 6 weeks into your pregnancy, with over 99% accuracy. Compared to theories based on ultrasound images like skull theory, these tests provide a much more reliable way to determine whether your baby will be a boy or a girl.

Traditional ultrasounds

Ultrasound scans are one of the most common ways to predict the sex during pregnancy. By examining the baby’s anatomy during a scan, providers can often identify whether you’re having a boy or girl. Ultrasound accuracy improves as pregnancy progresses—around 70% at 12 weeks, 95% at 14 weeks, and about 98% at 16 weeks. A standard 20-week anatomy scan is typically the most reliable ultrasound-based method.

Amniocentesis and CVS

Procedures like amniocentesis and chorionic villus sampling (CVS) can also determine the sex of your baby with close to 100% accuracy. However, these tests are invasive and are usually performed to screen for genetic conditions—not for sex prediction alone. Because they involve sampling amniotic fluid or placental tissue, they carry minimal risks and are typically recommended only when medically necessary.

Getting ahead of your baby’s sex with SneakPeek

Get answers sooner than ever with the SneakPeek Early Gender Test.

This simple, safe test can be taken as early as 6 weeks into your pregnancy—weeks before your sex reveal 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 sex soon after your kit is received.

What’s even better than finding answers early? Being confident in those answers. The SneakPeek Early Gender Test has proven to be over 99% accurate1 in laboratory studies.

Find out what determines a baby’s sex and learn your baby’s sex sooner than ever with the SneakPeek Test!