What is the nub theory and does it actually work?

Nub theory: what is it and does it work?

by Bryony Firth-Bernard |
Updated on

There’s so many different theories to help you try and determine the sex of your baby, and one of those is the nub theory.

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 the nub theory?

The nub theory is all to do with all that age-old guessing game of whether you'll give birth to a boy or a girl. 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.

How does the nub theory work?

The idea is that you can use your 12-week scan (which determines your due date) as a tool for predicting the sex of your baby. From around 12 weeks to 14 weeks you'll be able to clearly see your baby's nub, also called the genital tubercle. Both males and females have this nub, which will become the penis if it is a boy, or the clitoris if it is a girl.

By comparing the angle of this nub in relation to the spine, you can predict your baby's sex. For example, if the nub is angled at 30 degrees or more it is likely a boy, if it is under 30 degrees it is likely a girl. Predicting the angle of the dangle is tricky to get completely right though, even for the experts, which is why you're not normally told baby's gender until 20 weeks of pregnancy.

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 also notes that scanning is first and foremost for safety purposes only, as well as being a tricky method that 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!”

How accurate is the nub theory?

There is research that suggests some truth to this method, particularly in predictions made after 14 weeks gestation. The overall success rate in the first trimester group (11–14 weeks) was 75%, although it is worth noting that there were some scans where a prediction could not be made.

Results were less accurate for fetuses younger than 12 weeks, with an overall success rate of 54%.

However, Rachel advises caution when it comes to relying on this method for determining 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.

“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!”

If you are given a gender prediction at your scan, be aware that results vary depending on sonographer experience, fetal age and fetal gender. At 13 weeks gestation and above you may be able to get an opinion from your sonographer, but don't take it as gospel. Even the position that baby is lying in can effect the accuracy of predictions.

Predictions prior to 12 weeks are discouraged, and some sonography departments will actually refuse to give you a prediction under a certain gestational age.

Should I try the nub theory?

Understandably, you may be excited to glean any information on your upcoming arrival that you may be tempted to try nub theory for yourself. 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.

While it can be a bit of harmless fun to attempt, we recommend not taking the nub theory too seriously, though the accuracy rate sounds pretty good on paper, 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 theoryand our Chinese Gender Predictor Tool. Or what about these 26 pregnancy old wives' tales that hint at your baby’s gender?

But don't rely on these theories to pick your nursery colours. If you're after an accurate prediction it's better to wait until your 20 week anomoly scan and a medical professional has confirmed your baby’s sex.

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