Can Nub Theory Predict if You’re Having a Boy or a Girl?

As soon as you share your big baby news, you’ll hear it: “So are you going to find out what you’re having?” So many people want to find out baby’s sex (whether it’s their baby or not!), it’s no wonder there’s a seemingly endless array of legitimate and not-so-legitimate ways to do just that.

Nub theory, which relies on your sonogram image from an early pregnancy ultrasound, is one more gender prediction method to add to the mix. But before you give this one a try, learn how accurate the nub theory method of gender prediction really is.

What is nub theory?

Nub theory is a method some people use to predict a baby's sex. During the first part of pregnancy, both male and female fetuses have identical (and as-yet, not fully formed) genitalia known as a genital tubercle. This “nub” serves as the foundation for (you guessed it) nub theory.

The gential tubercle looks the same in both sexes at first, but around week 7 or 8 of pregnancy, things begin to change and baby’s reproductive system begins to take shape. 

If you’re having a boy, testosterone production begins, causing the penis to develop. Meanwhile, the absence of testosterone causes the clitoris to form in a baby girl. Even then, the penis and the clitoris will remain about the same size until about 14 weeks of pregnancy. 

Nub theory, however, hypothesizes that you can still predict a baby’s sex by looking not at the size but the angle of the tubercle. The theory goes that the “angle of the dangle” can predict a future baby boy or baby girl. But keep in mind that unless you have a trained scientist or doctor performing this baby sex predictor test, its accuracy is questionable at best.

When to use nub theory

People who use nub theory say you can try it as early as 12 weeks of pregnancy. That’s later than noninvasive prenatal testing (NIPT), which can reliably detect your baby’s sex and other info as early as 9 weeks. 

But not all pregnant moms get this genetic test, which can make nub theory sound more appealing. Otherwise, you’d typically wait until around the 20-week mark for the second trimester ultrasound  — when the reproductive organs have developed more fully — to find out the sex.

Nub theory for boys

To use nub theory to predict whether you’re having a boy or a girl, you’d technically need to measure the angle of the “nub” relative to your baby's spine.  If that angle is 30 degrees or more when looking at your baby from the side, you're supposedly having a boy.

If you don't want to bother with exact measurements (and flashbacks to middle school math), you can guesstimate that an upward-pointing tubercle might (operative word: might) mean you’re pregnant with a boy.

Nub theory for girls

If you precisely measured the angle of the tubercle compared to your baby's spine, an angle of 10 degrees or less supposedly indicates you're having a girl, according to nub theory. Though again, it’s far from foolproof, especially when performed by someone with no scientific or medical background.

If you prefer to simply look at the nub on an ultrasound image, check to see if it's pointed downward or is parallel with the spine, which, the theory goes, could mean you're having a girl.

How accurate is nub theory?

Nub theory is about 97 percent accurate, but only when performed by a trained scientist or doctor at 13 weeks or later, research indicates. But if said trained professional tries nub theory at 11 weeks of pregnancy, it’s only around 70 percent accurate, and when done at 12 weeks, it's about 85 percent accurate. Additionally, it's more common to mischaracterize a baby girl as a baby boy than the other way around.

Again, be aware that the scientists who conducted these studies carefully measured the “angle of the dangle.” Expecting parents who aren't scientists or mathematicians may not get as precise measurements even if they do pull out a protractor. Plus, if the angle measures between 10 and 30 degrees, nub theory doesn’t provide much clarity and won’t give you a definitive answer.

Like many other non-scientific gender predictor tests, the nub theory is just a guess and not a solid way to find out the sex of your baby.

If you want to know for sure, ask your practitioner about genetic testing such as NIPT, which is about 95 percent accurate between weeks 7 and 12 and 99 percent accurate after week 20. Or you can wait until your 20-week anatomy scan during the second trimester for another more reliable way to learn what you’re having.