Skull theory: can you really predict your baby's sex at 12 weeks?

What is skull theory? How does it work?

"Skull theory relies on an assessment of an ultrasound image of a baby's facial bony features and the skull to predict your baby's sex," says Christoph Lees, Professor of Obstetrics at Imperial College London and a faculty speaker for the International Society of Ultrasound in Obstetrics and Gynaecology.

There are key differences in the features of the face bones and skull of a grown man and a grown woman – so, the theory goes, if you look at your baby's skull on a pregnancy ultrasound pic, you can spot the features that will tell you if your baby is a boy or a girl.

Skull theory fans say you can tell a boy baby from a girl baby from the following signs:

• Shape and size of skull: a boy's skull is blockier and bigger than a girl's skull, which is smaller, rounder and tapers more at the top.
• Forehead: a boy's forehead is sloping while a girl's forehead is more vertical.
• Temporal ridge: the ridge that runs from the outer side of each brow to the back of the skull is squarer and more prominent for a boy than a girl.
• Brow ridge: the ridge above a girl's eye sockets is sharp, while for a boy it's more rounded.
• Cheekbones: more prominent on a boy than a girl
• Chin: a boy's lower jawbone is squarer then a girl's, which is rounder.

Skull theory: examples for boy and for a girl

The four pictures shown above, shared by different members of our MadeForMums Community, are all from 12-week pregnancy ultrasound dating scans.

The two pictures on the left are of boy babies; the two pictures on the right are of girl babies. (Each baby has since been born.)

If you look closely at the pictures, it does seem as though the boys in the pictures on the left have bigger, squarer skulls than the girls on the right, and that the girls have more vertical foreheads.

But, of course, as Gill Harrison, professional officer for ultrasound at the Society and College of Radiographers, points out, we're looking at scan images of babies in slightly different positions and shown at very different scales. "It's difficult to pinpoint these kind of differences," she says, "when these images really aren't comparable."

How accurate is skull theory?

It's no more accurate than a straightforward guess, we're afraid.

And that's because the neonatal skull (the skull of a baby in the womb) is not yet fully formed and, even if it was, the differences between male and female skulls are only visible in adults.

"Until you get to maturity or at least puberty," says Kristina Killgrove, a bioarchaeologist, science writer and research scholar at the Ronin Institute, "you just don't get these sexually dimorphic features of the skull in males and females."

In other words, while there may well be clear differences between a grown male and grown female skull, you wouldn't be able to see those differences in the skull of an unborn baby.

Professor Lees agrees, saying, "There is no scientific evidence for this theory at all."

Pics: MadeForMums Community members

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