We know, pregnancy can be a serious vocabulary minefield. From the time you first hear the phrase ‘nub theory’ to the moment you have your baby (and then for years after, while they’re growing up!), you’ll no doubt hear hundreds of other seemingly incomprehensible words and expressions.

But what exactly is nub theory? Does it work? Is it really accurate? We’ve got all the information you need to demystify this peculiar phrase…

What is nub theory?

The nub theory is all to do with all that age-old guessing game: will you have a girl or a boy? Essentially, nub theory is just another way to predict a baby’s gender.

“Sometimes referred to as looking at the ‘angle of the dangle’, some people claim that when you look at an early scan of the baby’s genital area, although both girls and boys have identical-looking little bobbles (or ‘nubs’), the male ‘nub’ sticks up at a different angle from the female ‘nub’,” says Rachel Fitz-Desorgher, a midwife with 30 years of experience in infant feeding, active births and working with parents.

Is nub theory accurate?

Like many other methods of guessing the gender, there is not enough good quality evidence to show that looking at the angle of the ‘nub’ will give you a reliable chance of correctly predicting the gender.

“Using the nub theory may be no better than swinging a coin over your tummy, looking at the shape of your bump, analysing your cravings or counting how many times you need to shave your legs in a month,” says Rachel.

“As the mum of four boys, my friends tested every one of their favourite gender-prediction theories on me and I can say categorically that – just like the scientific evidence tells us – none of them worked any better than 50:50. So toss a coin or just guess – it will work just as well!”

How does nub theory work?

People who believe in nub theory will usually say that you can tell the gender from looking at the 12-week scan photo, according to Rachel.

“When you look at a 12-week scan, you can see many things if you are trained to do so,” she says. “But even the very best sonographer will not claim to be able to tell you the sex of your baby with any certainty. The most accurate time to determine the sex of your baby is when your little warm bundle has safely reached your arms!”

Rachel adds that scanning is a tricky method and can lead to unexpected results.
“Scanning is a medical procedure done to confirm pregnancy and exclude abnormalities and the NICE guidelines only support the NHS providing them for this reason,” she says.

“Many parents, though, go into the room treating the scan as a nicety and want more info than the sonographer can actually give. Sonographers are put under a lot of pressure to look for gender and the sonographer is actually supposed to be concentrating on checking that the baby has all its arms and legs!”

Should I try the nub theory?

Understandably, you may be so incredibly excited to glean any information on your upcoming arrival that you may be tempted to try nub theory for yourself. And if you go online, you’ll find hundreds of parent-to-be sharing scan pictures and videos, asking for other people’s thoughts on the gender of their child. So while it can be a bit of harmless fun to attempt, we recommend not taking the nub theory too seriously – especially considering even the experts can get it wrong! 

Have a go at the nub theory 

Take a look at the following scan and see if you can guess the gender of the baby, using the nub theory…


Zara is having a bouncing baby boy! Did you guess right? 

What other gender theories exist?

If you fancy having a go at some other theories which aim to reveal your baby’s gender, take a look at the skull theory and our Chinese Gender Predictor Tool. Or what about these 26 pregnancy old wives’ tales that hint at your baby’s gender?

Rachel’s Fitz-Desorgher’s book Your Baby Skin to Skin: Learn to Trust your Baby’s Instincts in the first year is out now. 

Read next on Mother&Baby: