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Skull theory is a gender prediction method that some claim can show you if you're having a boy or a girl from a 12-week ultrasound scan picture.
Like the Nub Theory (which looks at the direction the genital tubercle is pointing) or the ancient Chinese birth chart (opens in new tab) (which uses the mother's age and the month of conception to guess a baby's sex), the Skull theory is another fun method to try and predict your baby's gender before your 20-week pregnancy scan (opens in new tab) - similar to the Ramzi theory.
Using your scan photo at just 12 weeks, the Skull theory - also know as the skull gender theory - looks at the shape and size of a baby's skull to determine whether you'll have a boy or a girl.
But is this just another old wives' tales about pregnancy or does this gender prediction method really work? Just remember this test, just like all our baby gender predictor tests (opens in new tab) is just for fun!
What is skull theory?
The skull theory is centred on an analysis of the cranial differences between men and women. Those who back the skull theory believe that such distinctions are visible in the womb.
This means that it might be possible to decipher whether a baby is male or female by studying the the shape of their developing skull in a 12-week scan photo.
How do male and female skulls differ?
The major difference between male and female skulls is that male skulls are more likely to be bigger and heavier.
The Smithsonian Museum of Natural History observes (opens in new tab): "Within the same population, males tend to have larger, more robust bones and joint surfaces, and more bone development at muscle attachment sites."
In terms of weight, male skulls tend to be heavier than those of females because the bones in a man's skull are said to be thicker too.
When it comes to the form or shape of the skull, male foreheads are lower and more sloping. Whereas a woman's forehead tends to be more vertical.
A comparison of the brow ridge - formally referred to as the 'supraorbital margin' - also yields an interesting observation. For men, this area is curved while for women, it's sharper in appearance.
Below the lower eye ridge is the zygomatic bone. More commonly known as the cheekbone, this tends to be more pronounced on the skull of a male. The same can be said of the superciliary arch, the ridge on the frontal bone above the eye sockets.
Another structural difference can be seen at the lower jawbone. A man's is squared, while a woman's has a rounded appearance.
It's also worth taking a look at the gonion - the point where the bottom of the lower jaw curves upward toward the ear. If it appears to be flared out or sharply angled on the scan, this might be an indicator that you're expecting a boy.
Skull theory’s accuracy
No gender prediction method is 100% accurate. Even the Ramzi Theory, which some claim is accurate in up to 97% of cases, isn't going to give you the right answer every time. If you want to know the gender of your baby for certain, you're best waiting for the results from your 20 week scan.
Working out your baby's gender based on the shape of their skull in your 12 week scan might be tricky. This is because, as The Smithsonian points out, the differences between the skulls of boys and girls is only 'subtle' at a young age:
"Sex-related skeletal features are not obvious in children's bones. Subtle differences are detectable, but they become more defined following puberty and sexual maturation."
In 2015 (opens in new tab), Scott Donald Haddow from the University of Copenhagen described the reliability of gender prediction methods using skull formation as "dubious" and many other researchers agree that it is impossible to distinguish between male and female skulls until late adolescence - around 18 years of age.
However, thegenderexperts.com (opens in new tab) report that the Skull theory has a range of accuracy that spans between 70-95%.
Although they also point out that there is a lack of scientific research into the Skull theory. "Statistical and quantifiable research on this theory still needs to be scientifically backed."