Nub theory: guess your baby's sex at 12 weeks

You've had the dating scan, the picture's in your hand, and somebody has told you there's a way to read boy or girl from it, eight weeks before the NHS will tell you. That's nub theory.

Here's how it works, how to look at your own scan, and a straight answer to one of the searches that bring people here most, which is whether the nub theory can be wrong. It can. It's a bit of fun with a protractor, and worth doing on exactly those terms.

The facts underneath it:

• The NHS dating scan is offered at around 10 to 14 weeks. It's for your due date, how many babies there are and checking development. Sex isn't on the list.
• Finding out the sex isn't part of the NHS screening programme. You may be able to at the 20-week scan, between 18 and 21 weeks, and it depends on your hospital's policy.
• Even at 20 weeks, the NHS says it's not possible for the sonographer to be 100% certain.
• The 30 degree rule is the theory's own. No NHS page, leaflet or midwife will measure it for you.
• Boys and girls both have a nub at this stage. The theory is entirely about which way it's pointing, on a picture taken at whatever angle your baby happened to be lying.

What is the nub theory?

There are plenty of old wives' tales for guessing the sex. Carrying low means a boy, craving sweet things means a girl, and so on. Nub theory is the one that claims to beat them, because it uses something real on a real picture.

At around 11 to 13 weeks every baby has a small bump between the legs, which the forums call the nub and the textbooks call the genital tubercle. The theory says its angle gives the game away.

If the nub points up from the line of the spine at more than about 30 degrees, it's called a boy. If it lies flatter than that, roughly parallel with the spine, it's called a girl.

That's the whole theory. Everything else is arguing about angles.

How to read the nub on your 12-week scan

You need the profile shot, with the baby lying flat and side on, so the spine shows as a clear line along the back. That's the standard dating scan picture, so start there.

1. Find the spine and follow it down to the bottom of the torso.
2. Look just below where the legs start for a small stalk or bump. That's the nub. Don't confuse it with the umbilical cord, which comes out of the tummy further up and is the classic mix-up.
3. Imagine a line along the spine and a line along the nub, and guess the angle between them. People do download protractor apps for this.
4. Over 30 degrees, call it a boy. Flat, call it a girl. Somewhere in between, post it on a forum and let strangers fall out over it.

A girl nub, by the theory, sits almost horizontal, running along the same line as the spine rather than lifting off it.

A boy nub lifts away from the spine at a clear upward angle, and the steeper it is, the happier the boy camp gets.

Can you find out the sex of your baby at 12 weeks?

Not from the NHS, and not reliably from anyone. The dating scan, offered at around 10 to 14 weeks according to the NHS 12-week scan page, is there to work out how many weeks pregnant you are and your due date, to check whether you're expecting more than one baby, to check the baby is growing in the right place, and to check development. The sonographer is measuring, not sexing.

The NHS is clear that finding out the sex isn't offered as part of the national screening programme. Its ultrasound scans page says you may be able to find out during the 20-week screening scan, which happens between 18 and 21 weeks, but that it depends on your hospital's policy, and that it's not possible for the sonographer to be 100% certain.

Ask at the start of the scan, not as the gel's being wiped off, and be ready for a "probably".

It does happen that a 20-week "girl" arrives a boy. It's rare, and the reason is in that NHS line: a baby with crossed legs or facing the wrong way doesn't give a clear view, and a good sonographer will say so rather than guess.

Can the nub theory be wrong?

Yes, often, and here's how.

• The baby isn't lying flat. A slight tilt towards or away from the probe changes the apparent angle of everything, nub included.
• The picture is at the wrong angle. The sonographer is chasing measurements, not a nub shot, so the still you get is whatever was on screen when they pressed print.
• It's early. The nub changes over these weeks. By the theory's own logic a boy's nub may not have tilted up yet at 11 weeks, so it reads as a girl.
• It's the cord. The umbilical cord gets mistaken for a nub all the time.
• It's a guess to begin with. Half of all guesses are right by chance, which is why every method ever invented has fans.

You'll see accuracy figures by week on forums. They come from people reporting their own result after the birth, and people mostly report the times they were right. The wrong guesses go quiet.

Nub and skull theory together

Skull theory reads the shape of the head on the same picture: blocky and sloping for a boy, rounded and tapered for a girl. People run both and call it a second opinion. It's two guesses, and when they agree it still isn't proof. The skull theory guide has the boy versus girl checklist and the same honest verdict.

Questions we get asked

What is a nub on a baby scan?

The small bump between the baby's legs at around 11 to 13 weeks. Its proper name is the genital tubercle, and every baby has one at that stage. Nub theory is about the angle it makes with the spine.

Is nub theory more accurate at 13 weeks than at 12?

By its own logic, yes, because the nub is easier to see and has had longer to tilt, and forum regulars will tell you 11 weeks is too early. That makes it a better guess, not a reliable one. At 13 weeks the real answer is still seven weeks away.

Can I ask the sonographer at the 12-week scan?

You can ask anything. Most will tell you the dating scan isn't for that: the NHS doesn't offer finding out the sex as part of screening, and at 12 weeks they're measuring for your dates and checking development. The 20-week scan is when to ask, if your hospital's policy allows it.

Can the umbilical cord be mistaken for the nub?

Yes, and it's one of the commonest ways the theory goes wrong. The cord comes out of the tummy, the nub sits between the legs, and on a small grainy picture they can look alike.

What if I can't see a nub on my scan photo?

It means the shot isn't a clean side profile, not that you're having a girl. Some people post the picture on a forum for a second opinion. Or you wait for the 20-week scan, which is what the nub is quietly suggesting.

Come back and tell us if you were right. The comments are open, and the honest ones are the wrong guesses, so post those too.

The pregnancy collection is there whenever you're ready, and it doesn't care what the nub said.