The Nub Theory


"I've waited 12 weeks already, how much more?"
- Very impatient mama who needs to know more!

The basics of this theory are simple and straight forward (no pun intended), it's all about the angle of the genital tubercle. The male nub is usually angled upwards 30 degrees or more in relation to the spine, while female nub is either parallel or less than 30 degree from the spine. Several studies have been conducted and have found that, in controlled settings, nub theory ranges between 98% and 100% in accuracy. That is great odds for knowing your little one's gender before your doctor can confirm it!

For more information, here are a couple of the most recent studies conducted:
Ultrasound evaluation of fetal gender at 12–14 weeks
Fetal gender assignment by first‐trimester ultrasound

Want to read your own scan?

Both male and female start forming their genitalia in the very early weeks after conception, but it isn’t until 11-12 weeks that the differences can be seen. Although outward genitalia isn't fully formed until after 16 weeks, the angle of the genital tubercle can be seen from 11 to 15 weeks to determine gender. There are two main struggles when trying to read your own ultrasound. Perhaps the trickiest part of reading your own ultrasound is being able to locate the nub. It is usually right under the legs, but because every image is different and babies like to move around during ultrasounds, it takes an expert eye to pinpoint it for sure. The second struggle is seeing more than just the basic angle, as at 11 and 12 weeks the male nub might still be parallel, but with an expert eye the scrotum can began to be seen.

How read your own scan?

In order to be able to make a gender prediction the ultrasound must display a profile view of the baby, and the nub must be visible, this can be a challenge because babies move and good profile views are hard to get. If you do achieve this image, here are the steps for nub method:

-Find the spine. The angle of the spine serves as a baseline to compare with the angle on the nub, which is our next step.

-Find the nub. It should be a defined white line along where the legs of the baby should be.

-Find the angle. Comparing the direction of the spine with nub, see if the angle is more or less than 30 degrees.

As mentioned above, in addition to the angle there are other factors that also need to be taken into consideration. Some believe that if a nub has some forking on the tip it is a male nub. Baby Gender Pros does not believe this to be true as we have seen make forked female nubs, however it is a marker that we look at closely. Shadowing above the nub or stacking are also markers that can be deciding factors towards our prediction.