The Nub Theory Explained: Can You Really Tell Your Baby’s Gender at 12 Weeks?

Written By Edina Hockley

If you’re in your first trimester and just can’t wait to find out your baby’s gender, you’re not alone! Many people start googling “Can you tell gender at 12 weeks?” the moment they see that first flickering heartbeat. And I get asked all the time in the BellyView studio & in clinic.

But can you really tell? Well… sometimes yes!
That’s where the Nub Theory comes in.

What Is the Nub Theory?

The Nub Theory is an early gender prediction method based on the angle of your baby’s genital tubercle. It’s also known as the “nub.”

Around 11 to 13 weeks of pregnancy, all babies have this small, unformed structure that will eventually develop into either:

  • the clitoris and labia, or

  • the penis and scrotum.

By looking closely at the angle of the nub in relation to your baby’s spine, experienced sonographers can often make an educated guess about your baby’s gender.

This angle is sometimes referred to as the “angle of the dangle.”

How the Nub Theory Works

When your baby is around 12 weeks, the nub starts to take shape.
Here’s the general rule of thumb:

  • 💗 If the nub is angled less than 30° from the spine, it’s likely a girl.

  • 💙 If the nub is angled more than 30° upward, it’s likely a boy.

The angle of the nub for a boy and a girl.

But it’s not just about the angle, we also look at:

  • The length and shape of the nub

  • The position of the bladder

  • The clarity of the ultrasound image

  • The baby’s position during the scan

When all these factors line up, the nub theory has been reported to be up to 92% accurate.
I’m not sure how accurate that percentage is though…

What Affects Nub Theory Accuracy?

While the science behind the nub theory is relatively strong, a few things can influence how accurate the prediction is:

  • 🕐 Gestational age: Before 12 weeks, the nub hasn’t developed enough to read clearly.

  • 👶 Baby’s position: Your baby needs to be in a clear side (profile) view.

  • 💧 Mothers BMI and clear amniotic fluid

  • 📸 Image quality: High-resolution equipment and an experienced sonographer can make a difference.

For many expecting mums, the nub theory isn’t just about guessing, it’s about connection.
It adds a little spark of excitement to those early weeks and helps you start imagining who’s growing inside. I know it can be hard to wait!

At BellyView Ultrasound, I love having a guess for you but it is certainly not guaranteed!

How Accurate Is the Nub Theory?

Studies show nub theory predictions can be:

  • 80–92% accurate after 12 weeks +4 days

  • Less reliable before 12 weeks (the nub hasn’t differentiated yet)

It’s a fun, science-based prediction, not a guarantee. But when combined with professional imaging, it can sometimes be correct.

It’s better to wait for this below! 👇🏻

For more accuracy, I definitely recommend booking an early gender reveal ultrasound from 14 weeks onward. Baby’s nub has developed sufficiently enough then to have a clearer evaluation. We can be more sure of whether you’re having a boy or girl after 14 weeks.

Edina Hockley
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