What Is the Nub Theory?
The nub theory (sometimes called "angle of the dangle") is a method of predicting a baby's sex by analysing the angle of the genital tubercle — a small nub visible on ultrasound at 11–14 weeks gestation.
At 11 weeks, male and female embryos look almost identical. However, by examining the angle at which the nub protrudes relative to the baby's spine, experienced observers can predict gender with up to 97% accuracy at 13+ weeks.
The Science Behind It
All embryos begin with an identical genital tubercle. Around week 11:
How to Read the Angle
To apply nub theory, you need a profile view of the baby (side view) showing the spine and the bottom of the torso.
Step 1: Identify the spine
Find the white line of the baby's spine running along their back.
Step 2: Find the nub
Look for the small protruding area between the baby's legs.
Step 3: Measure the angle
Draw an imaginary line from the nub, parallel to the spine.
Accuracy by Week
| Week | Accuracy |
|---|---|
| 11 weeks | ~70% |
| 12 weeks | ~85% |
| 13 weeks | ~97% |
| 14 weeks | ~99% |
Try Our Free Nub Theory Analyzer
You don't have to guess alone. Our Nub Theory Analyzer lets you upload your 12-week scan and get an AI-assisted angle reading instantly — free, no payment required.
Important Disclaimer
Nub theory is not a medical determination of sex. It's a fun, statistically-backed method for curious parents. For a confirmed gender determination, speak to your healthcare provider at your 20-week morphology scan.