Skull Theory: Can You Predict Baby's Sex from the Skull?

Skull theory claims you can tell a baby's sex from the shape of its skull on an ultrasound image. The supposed signs are: a squarer, blockier skull with a more prominent brow and flatter jaw suggests a boy; a rounder, smaller skull with a more pointed jaw suggests a girl.

It is usually applied to the same 12-week scan image used for nub theory — but instead of reading the genital tubercle angle, it looks at the shape of the baby's head. Some practitioners combine the two, using skull theory as a "secondary clue" alongside the nub angle.

The theory circulates widely on forums and social media, often alongside labelled diagrams showing "boy skull" vs "girl skull" shapes. The question is whether any of it is backed by science. For where skull theory sits among the popular methods, see our nub vs Ramzi vs skull comparison.

No. There is no peer-reviewed research supporting skull theory. A search of the medical literature turns up no published study that has tested and confirmed the method. This is not a case of "the evidence is weak" — there is simply no evidence at all.

Contrast that with nub theory, which has been tested in multiple peer-reviewed studies over more than two decades: Efrat et al., 1999 (n=172), Efrat et al., 2006 (n=656), Manzanares et al., 2016 (n=672), and a review of 2,314 pregnancies. Nub theory has evidence; skull theory has none.

The biological reason is straightforward and well-established: human cranial sexual dimorphism does not appear until puberty.

The skull features that differ between adult men and women — such as a more prominent brow ridge, squarer jaw, and larger cranial capacity in males — develop under the hormonal changes of adolescence. Before puberty, children's skulls show minimal sexual dimorphism. And a fetus, of course, is decades away from puberty.

This means there is genuinely nothing to read on a prenatal ultrasound. The differences that skull theory claims to spot simply do not exist at this stage of development (Healthline). Any pattern a reader thinks they see is a product of interpretation and image artefact, not anatomy.

If the sex-related differences do not exist, why do some babies' skulls look blockier and others rounder on a scan? The answer is image artefact.

A 2D ultrasound captures a single slice through the baby. The apparent shape of the skull on that slice changes dramatically depending on:

• The angle of the probe: a slightly different angle can make the same skull look squarer or rounder.
• The baby's position: a head tilted forward or back changes the profile dramatically.
• The depth of the slice: a cut through the middle of the skull looks different from a cut nearer the side.

So two scan images of the same baby can show what looks like a "boy skull" and a "girl skull" depending on the angle. This is why skull theory is not just unsupported — it is fundamentally unreliable. The shapes it reads are artefacts of imaging, not biological signals.

If you want to predict your baby's sex from the scan you already have, use nub theory instead. It reads the genital tubercle angle — something genuinely related to the baby's sex — and has real peer-reviewed evidence behind it (roughly 85–90% at 12–13 weeks). See our complete guide to nub theory and our boy vs girl nub guide.

If you want the earliest reliable answer, NIPT (a maternal blood test from about 10 weeks, ~96–99% accuracy) is the gold standard (Devaney et al., JAMA 2011). See our NIPT vs nub theory comparison.

NubHub reads the nub — not the skull — on your 12-week scan, pairing AI with a human double-check. Results are back in about 2 hours, the cost is under £40, and every reading is backed by a money-back guarantee. See how our AI works.