The angle of one small bump
Nub theory is the method most people have already half-heard of. It looks at a tiny structure that's visible on almost every 12-week profile scan, and reads its angle.
What it looks at
Early in pregnancy, every baby has a small bump called the genital tubercle, visible on a clear side-on (profile) scan image. Nub theory looks at the angle that bump sits at, measured against the line of the spine.
The rule of thumb
The popular version says a bump sitting at more than roughly 30 degrees off the spine tends to get read as boy, while one lying closer to flat, more parallel to the spine, tends to get read as girl. It's a rule of thumb, not a formula, and the angle can be genuinely difficult to judge on an ordinary phone photo of a scan printout.
Why it isn't certain on its own
The tubercle looks broadly similar in both sexes at 12 weeks, and only becomes clearer with more development. The angle in the photo also depends heavily on the exact position the sonographer caught baby in, and on how far along the pregnancy actually was on the day, which is why we ask for your scan date. On a clear, well-angled image nub theory can be a strong signal. On a blurry or awkwardly angled one, it can't be read with any confidence, and that pulls the overall score down.
How it feeds the score
Nub theory is one of Kay's two inputs into your combined percentage. Its weight in your particular reading depends on how clearly the angle can actually be judged on the image you send us.