Nub theory explained: how this sex prediction test works

Usually, the 20-week scan is the earliest point at which you can find out your baby's sex - although some expert sonographers reckon they can tell as early as 16 weeks.

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But now a theory claims that you can tell whether you're expecting a boy or girl at just 12 weeks pregnant. If you're desperate to find out what you're having, it sounds amazing - but is it too good to be true?

The so-called 'nub theory' is based on studying the baby's genital area at 12 to 14 weeks. At this stage, both boys and girls have a penis-like 'nub' called the genital tubercle. And although their genitals look the same, some experts think they can use the subtle differences to tell the baby's sex.

So what is the nub theory?

The nub theory is all based around the angle of your baby's nub.

If the nub is pointing upwards, at an angle of more than 30 degrees to the spine, the baby is likely to be a boy. If it's pointing downwards, straight out or upwards at an angle of under 30 degrees, the chances are you're having a girl.

Some people also say that the length and shape of the nub can give you a clue about your baby's sex, with a forked shape at the end of the genital tubercle suggesting it's a girl and a shadow at the end indicating that it's a boy, but there's no hard evidence to support this.

What does the nub look like?

On the ultrasound pictures below, we've highlighted the baby's nub. It may not be easy to spot on your scan photos: it depends on getting a really clear side-on photo that shows both the nub and your baby's spine, so you can look at the angle between the nub and spine.

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The signs you're having a girl

If you didn't find out the sex of your baby at your 20-week scan, you'll be spending the rest of your pregnancy wondering if you're growing a little boy or girl. There are plenty of old wives' tales out there to help you try and predict your baby's sex; here are 18 signs that you might be carrying a girl.

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How accurate is the nub theory?

The jury is out on whether the 'angle of the dangle' is a reliable way to work out your baby's sex. A round-up of studies found that:

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• At 11 weeks, the accuracy rate is 48%
• At 12 weeks, the accuracy rate is 91%
• At 13 weeks, the accuracy rate is 94%

But - and it's a big but - these results are based on scan pictures taken at exactly the right angle, and interpreted by experienced sonographers who have studied thousands of images.

It's much harder for an inexperienced eye to be able to identify a baby's sex from a grainy ultrasound photo.

Some clinics advertise themselves as experts who can interpret scan photos, but there's no guarantee that their prediction will be correct. It can be difficult to work out the exact angle, especially if it's between 10 and 30 degrees, and before 12 weeks, the angle of the nub can change from day to day.

Our verdict? It's fun to guess, but don't go out buying pink or blue yet!