Nub theory – can you predict if you’re having a boy or girl?

Nub theory is based on your 12-week pregnancy scan pic: it says you can predict the sex of your baby based on the angle of their genital tube. So what's the science behind it - and how accurate is it?

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According to nub theory it’s possible to predict your baby’s sex at the 12-week scan. The idea, known as the ‘angle of the dangle’, suggests that you can tell the sex of your baby at your very first ultrasound.

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Nub theory explained by a doctor

“Nub theory is to do with what we can potentially see on the 12-week scan,” says our expert GP Dr Philippa Kaye.

“The purpose of the 12-week scan is a dating scan, to check how many babies we have in there, and that there is a heartbeat. They’re not to going to look at the genitals  at all.

“Before the genitals are formed there’s a bump or nub called the genital tubercle / tube.

“The basis of the theory is that this tube causes an angle with the lower part of the spine – and if that angle is less than 30 degrees it’s a girl, and if above it’s a boy.”

Nub theory – girl example

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This image was shared on our forum, and according to nub theory – it’s a girl – as you can see, the angle of the ‘nub’ is less than 30 degrees.

Nub theory – boy example

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This scan pic was shared on our forum by jwebster82.

We think the angle here looks less than 30 degrees, in fact, it’s flat – which, according to nub theory, means it’s a girl. But the baby turned out to be a boy.

Is nub theory scientifically proven?

No – there’s no solid scientific research into whether or not nub theory actually works, and that’s because most medics claim it’s just too early to accurately predict a baby’s sex at 12 weeks.

“There is no evidence that it works,” reiterates Dr Philippa. “It’s said it works 50% of the time – that’s as good as guessing – so I wouldn’t go for it.”

And anecdotally, it seems that nub theory ‘works’ for some and not for others. On our forum, mum Gazsgirl says: “Mine was right at 12 weeks, the nub was level and at my 20-week scan it was confirmed I am having a girl.”

While another of our forum members, Huxley, who has had some experience as a sonographer tells us: “I’m a radiographer with a bit of experience in sonography and I’m afraid I can’t tell from scan pics what the sex is.

“The best way is to take an image of between their legs at 17 weeks onwards and see from there! I just read about the nub theory, it’s interesting but I’m not sure I’d trust it till a lot more research had been done on it.”

And when we contacted the Royal College of Radiologists, a spokesperson advised that one of their obstetrics radiology leads there has said using nub theory to predict your baby’s sex is possible, but extremely difficult.

“He said no hospital trust routinely offers sexing at 13 weeks (when you’re talking about for nub testing), as it makes sense to wait and be more confident,” she went on to say.

“Unless they have a local policy against doing it (usually due to cultural demographics), most ultrasound departments will sex babies at 20 weeks, with the usual proviso that it is not fool proof.”

Can nub theory be wrong?

In a word – yes. In all honesty it’s a bit of a guessing game. To see the nub your baby needs to be lying on their side, not curled up and they need to be as flat as possible.

“Up until your 14th or 15th week scan, the female and male genitalia simply look too similar to 100% differentiate between the two. This can potentially lead to inaccuracies if a result is given before this point,” says Dr Geetha Venkat, who specialises in fertility treatments.

“It is very difficult to identify internal or protruding sexual organs until they are suitably developed. With this in mind, it is better to be patient and wait until the appropriate scan when discovering the sex of your baby, and remember the most important thing is that they are healthy.”

Podcast: How reliable are gender prediction tests?

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