The genital tubercle or “nub” is fetal tissue that will develop into a penis or a clitoris as the pregnancy progresses. Nub theory says if you can get a good look at this nub on an ultrasound, you may be able to determine the sex of a fetus at an early gestational age.

If you’re pregnant and impatiently counting down the days until your 18- to 22-week anatomy scan — the ultrasound that will give you all kinds of important info about your growing baby, including their biological sex — there’s a good chance you’ve fallen into the internet rabbit hole of sex-prediction theories.

While there, you may have come across something called “nub theory.” It gets a lot of attention for being a legit way to predict your baby’s sex much earlier than normal.

Videos and internet forums are full of people claiming to know how to analyze the early stages of a baby’s genital development to figure out whether that little appendage will turn into a boy part or a girl part.

There are even online companies offering to read your ultrasound results and “interpret” your baby’s nub for you. (For a fee, of course!)

But what, exactly, is nub theory — and is it a genuinely accurate way to predict your baby’s sex?

Nub theory revolves around something called the genital tubercle, which forms early in pregnancy on the lower abdomen of your baby. Eventually this tubercle, or “nub,” turns into a penis in male babies and a clitoris in female babies.

The idea behind nub theory is that if you can get a really good look at this nub, you can figure out which way it will go in the coming weeks.

Specifically, prospective parents are instructed to look at what the internet calls “the angle of the dangle.” (Yes, we just said that.)

In nub theory, the angle of the nub in relation to the spinal cord tells you everything you need to know about whether your baby’s nub will soon develop into a penis or clitoris.

According to proponents of nub theory, you can crack the code of your baby’s sex at a 12-week ultrasound. It’s true that between 8 and 9 weeks of gestation the genital tubercle begins to take shape, though it looks largely the same in both sexes until about 14 weeks.

Nub theory fans, however, claim that by 12 weeks the tubercle is different enough in appearance to be noted on an ultrasound.

To actually apply nub theory to your baby’s ultrasound, you need to catch them in a clear profile so the length of their spine is visible horizontally. From there, you would search for the nub, or a small protrusion, in between where your baby’s legs will form.

If your baby’s nub is angled higher than 30 degrees in relation to its spine, that indicates your baby is a boy, according to nub theorists.

Now, no one is saying to whip out a protractor to figure out the specific angle here, but obviously this is where nub theory gets a little muddy.

What exactly does 30 degrees look like on an ultrasound? We don’t really know, but if you were to draw a straight line on the ultrasound along the bottom half of your baby’s spine (where their butt is, basically), you can eyeball whether the nub is pointing clearly up away from that line or not.

If it is, it’s allegedly a boy.

On the flip side, if you compare the angle of your baby’s butt to the angle of its nub and it’s horizontally in line with the spine or pointing down toward it, that’s supposed to indicate that your baby is a girl.

Nub theory is a good name for this analysis, because it really is just that: a theory, without much evidence behind it. Anecdotally, some sites will tell you that the prediction is tremendously accurate.

To be clear, this isn’t a totally made-up thing. There actually are a few (older) studies suggesting that you may be able to determine a baby’s gender from an early ultrasound using the angle of the genital tubercle.

In a small study from 1999, researchers analyzed the tubercles of babies in 172 pregnancies, determining whether the angles were greater or less than 30 degrees. At 11 weeks, there was 70 percent accuracy in determining gender, and by 13 weeks, that number had jumped to more than 98 percent.

Similar results were found in a 2006 study, with a larger sample size of 656.

However, in a larger study from 2012, the accuracy was found to be much lower, though it did increase over time as the gestational age increased. This suggests that waiting longer to determine sex leads to more accurate results.

Most pregnant women undergo first trimester screening that includes an ultrasound and blood tests for chromosomal abnormalities like Down syndrome and trisomy 13.

This is usually done between 11 and 14 weeks gestation and includes the same ultrasound that nub theory proponents claim can be used to predict the baby’s sex.

Typically, the prenatal blood tests involved at this stage check for protein and hormone levels that can signal fetal abnormalities. But if you’re at risk for other abnormalities, especially sex-linked disorders like hemophilia and Duchenne muscular dystrophy, your doctor may suggest including a blood test that can detect a baby’s sex.

Nub theory is a fun way to make a slightly-better-than-random guess at your baby’s sex after your 12-week ultrasound. (Hey, it’s probably more accurate than peeing in a cup of salt water to see if you’re pregnant!)

But we don’t recommend committing to a gender-based nursery decor theme until you’ve had your full anatomy scan and a medical professional has confirmed your baby’s sex. Before that, nub theory is no better than a guesstimate.