After you’ve toughed out the TTC period, anxiously did the TWW, and finally got that BFP, you’re over-the-moon ecstatic that you’re about to be a parent.

Say what? About those acronyms…

  • TTC = trying to conceive
  • TWW = two-week wait (the time between conception and when you can take a home pregnancy test)
  • BFP = big fat positive
Was this helpful?

Whether this is your first time or you’re adding on to your family, you’re imagining what your new little one is going to look like. Will they have your eyes or your partner’s smile?

You might be too impatient to wait until your 20-week anatomy scan to find out if you’re having a boy or a girl. But there’s a rumor that you can use earlier ultrasounds to determine the baby’s sex with a nifty trick.

It’s called skull theory, and while some women swear by it, others view it as nothing more than an urban legend.

So, we’re going to get to the bottom of it.

Skull theory — also sometimes written as skull gender theory — is the belief that you can accurately predict the gender of your baby well before the 20-week scan by looking at your earlier ultrasound images.

According to the theory, the shape and size of a baby’s skull can determine whether you’ll have a boy or a girl.

While no one seems to be able to actually pinpoint where skull theory originated, anecdotally, it seems to be a fan favorite on pregnancy forums.

A casual internet search will send you down the rabbit hole of forums from around the world with moms posting early ultrasound scans and encouraging commenters to guess the sex of their baby — with varying degrees of success.

If you’re thinking of trying skull theory to determine your baby’s sex before your 20-week anatomy ultrasound, you’ll need to make sure that you get a very clear image from your 12-week scans.

However, “very clear” can be difficult — the baby’s position in the womb at the time of your scan can impact how well you see the skull.

According to skull theory supporters, you should try to get the baby clearly positioned in profile where the skull can be measured from front to back. But anecdotal research on various pregnancy forums shows that even with a clear ultrasound, it’s not always obvious (or unanimous among users who offer their opinion) whether you’re having a boy or a girl.

The general belief is that baby boys have bigger and blockier skulls than girls. More specifically, boys have a defined brow ridge, square chin, and more angled jaws. Plus a boy’s skull tends to have more prominent cheekbones.

In contrast to boys, baby girls have rounder chins with a wider angle to their jaws. Additionally, their foreheads are less sloping with smaller brow ridges.

Even supporters of skull theory state that the accuracy is only between 70 and 95 percent and that more research is needed to prove that this is a viable early gender test. And really, there is little to no evidence from peer-reviewed journals.

When we look at scientific experts in fields like anthropology and archeology, we begin to realize why skull theory is a great conversation starter — but shouldn’t be relied upon to genuinely determine a baby’s gender.

Many of the skull differences listed as the determiners for boys versus girls are actually only visible in adult skulls. In reality, those unique indicators don’t usually appear on the human skull until puberty at the earliest. These features are used to determine sex when excavating archeological sites and looking at human remains.

But in neonatal skulls, those differences aren’t truly visible, making skull theory an unreliable option.

So, if skull theory is a fun game but not reliable, what other options are there if you can’t wait until the 20-week anatomy scan to learn what you’re having?

A good answer is testing that coincides with the nuchal translucency (NT) scan, an optional test that is usually completed between the 11th and 13th week of pregnancy. The NT scan is a noninvasive test that’s primarily used to screen for any abnormalities with your baby’s development.

Specifically, this scan is performed to measure the size of the clear tissue — known as nuchal translucency — at the back of your baby’s neck. If there’s too much clear space, it could be a sign of a genetic condition such as Down syndrome or even chromosomal abnormalities that may be fatal to the baby.

But what many people might not realize is that the NT scan appointment can also include a blood test to further screen for chromosomal issues. This blood test can also accurately determine the sex of your baby.

Again, keep in mind that the NT scan and blood test is optional. You may need to specifically request it unless you’ll be older than 35 at the time of delivery or are otherwise considered at higher risk of having a baby with a health complication.

There are no shortages of non-medical myths that promise to accurately predict the sex of your baby.

While skull theory is one of the more novel options, we’ve all heard that the way you carry your baby during pregnancy or certain food cravings indicate a specific sex.

The truth is, there are only a handful of ways to accurately predict your baby’s gender, and they require something far more scientific.

If you opt to learn what you’re having before you give birth, keep in mind that the only (mostly) “goof-proof” options are an early blood test or at your 20-week anatomy scan. And be prepared: Even with a second trimester ultrasound, there are sometimes surprises!