Reading a silhouette
Skull theory took off on parenting forums and TikTok long before it had a name. It looks at the outline of baby's head on a side-on scan image.
What it looks at
On a clear profile shot, you can usually make out the slope of the forehead, the shape of the jaw, and how pronounced the browline looks. Skull theory compares those features against a rough pattern people believe shows up more often in one sex than the other.
The rule of thumb
The common version says a rounder forehead and a softer, less defined jaw tends to get read as girl, while a more sloped forehead and a more prominent brow tends to get read as boy. It's entirely visual and entirely subjective, there's no measurement involved the way there is with nub theory.
Why it isn't certain on its own
Skull shape at 12 weeks varies hugely from baby to baby regardless of sex, and the angle a photo is taken from can change a silhouette completely. There's no clinical study behind this one, it's pattern-spotting that spread because it's fun to do, not because anyone proved it works. Kay treats it as the softer of her two signals.
How it feeds the score
Because it's the more subjective of Kay's two methods, skull theory carries the lighter weight in your combined percentage. It mainly nudges the score when nub theory is already leaning strongly one way.