Skull theory: boy or girl from your 12-week scan photo?

The 12-week scan photo is on the fridge, it's in the family group chat, and somebody has already zoomed in on the head and announced "that's a boy". That's skull theory, and this is the most read page on our blog, by some way.

So here it is done properly: what the theory says to look for, how to look, and an honest answer on whether it works. Short version, it's a bit of fun. Keep it that way and you get eight weeks of happy arguing before the 20-week scan gives you a proper answer.

The facts underneath the fun:

• The dating scan is offered at around 10 to 14 weeks, the NHS says, to work out your due date, check how many babies there are and check development. Nobody in that room is looking at the sex.
• Finding out the sex isn't part of the NHS screening programme. You may be able to at the 20-week scan, which happens between 18 and 21 weeks, and it depends on your hospital's policy.
• Even then, the NHS says it's not possible for the sonographer to be 100% certain.
• Skulls don't look male or female until a person is grown, which is the plain reason the theory can't work at 12 weeks.
• A coin gets it right half the time. So does every gender theory ever invented.

What is skull theory?

Skull theory is the idea that you can tell a boy from a girl by the shape of the head on an ultrasound picture, usually the one from the dating scan at 12 weeks or so.

Boys, the theory goes, have a bigger, blockier skull with a heavy brow and a forehead that slopes back. Girls have a rounder head that tapers towards the top, a smaller brow and a pointier chin.

It borrows the language of adult skulls (brow ridge, frontal bone, gonion), which is the theory's whole problem. At 12 weeks you're looking at a tiny head on a photo the size of a bus ticket.

It lives on forums and in group chats, where somebody posts their scan and everyone votes.

How to do skull theory on your 12-week scan photo

You need the profile shot: baby side on, face to the left or right, spine running along the bottom. That's the standard dating scan picture, so most people have one. A face-on picture is no use, and a blurry one is no use either, which rules out a lot of them.

Then look at four things.

1. The forehead. Does it rise more or less straight up from the brow, or slope back towards the top of the head?
2. The brow. A heavy ridge over the eye socket, or a small sharp one?
3. The chin. Square and forward, or small and pointed?
4. The jaw. Follow the lower jaw back towards the ear. Does the corner flare out and turn up at an angle, or curve round softly?

Upright forehead, sharp brow, pointed chin, soft jaw: the theory calls that a girl. Sloping forehead, heavy brow, square chin, angular jaw: the theory calls that a boy. Two of each, and you're back to the coin.

Skull theory boy vs girl

Here's the full checklist the theory uses, side by side. Read the last row with the seriousness it deserves.

Skull theory girl, in a sentence: a neat rounded head that comes to a gentle point at the top, a small sharp brow, an upright forehead and a pointy little chin.

Skull theory boy: a big square head, a heavy brow, a forehead that slopes away and a jaw with corners.

Is skull theory accurate?

No. There's no scientific basis for it. Skulls don't develop distinct male and female features until a person is grown, and even then it's a judgement rather than a rule, with things like ethnicity muddying it. At 12 weeks the sonographer is measuring your baby for dates and checking development, and the picture you take home is a still from that, at whatever angle your baby happened to be lying.

You'll see big accuracy numbers quoted on forums. Treat them the way you'd treat a mate's football tips: people report the wins and forget the misses, and half of all guesses are right before anyone has looked at a forehead.

For comparison, the real thing isn't perfect either. The NHS says that if you want to find out the sex you may be able to at the 20-week screening scan, that it depends on your hospital's policy, and that it's not possible for the sonographer to be 100% certain. Legs crossed, baby facing the wrong way, and you get a "probably".

So play. Just don't paint the nursery on it.

Nub and skull theory together

People often run the two together for a "second opinion". Nub theory reads a different part of the same 12-week picture: the angle of the nub (the genital tubercle) against the spine, with steeper than about 30 degrees called a boy and flatter called a girl.

It has the same standing as skull theory, which is to say none, and it's at least as much fun. There's a full guide to nub theory, including the ways it goes wrong.

Two guesses that agree feel like proof. They're still two guesses.

Questions we get asked

Does skull theory work at 10 weeks?

You'd need a scan picture first, and the NHS dating scan is offered at around 10 to 14 weeks, so most people don't have one yet. If you've had an early scan, the same rules apply and so does the same honesty: whatever the forehead is doing, it's a guess.

Does skull theory work at 20 weeks?

The theory is built around the 12-week profile picture, but by 20 weeks you don't need it. That's the scan where you can ask the sonographer, if your hospital's policy allows it and your baby is lying the right way. The NHS says even then they can't be 100% certain.

What is the head theory or forehead theory?

Skull theory under other names. Forehead theory is the sloping versus upright part of it, and head theory is what people call it when they can't remember the word skull.

What's the difference between nub theory and skull theory?

Skull theory reads the shape of the head. Nub theory reads the angle of the nub between the legs against the spine. Both use the 12-week scan picture, both are folklore, and plenty of people do both and go with whichever answer they wanted.

Do I need a side-on scan photo for skull theory?

Yes. Everything the theory looks at is in profile: forehead, brow, chin and jaw. A face-on picture tells you nothing, and neither does a fuzzy one.

When you do know for sure, come back and tell us in the comments whether the skull got it right. Half of you will say yes, and that's the theory in one line.

The pregnancy collection is there for the buying stage, and none of it needs you to know the answer.