The Nub Theory: Early gender determination by ultrasound

If you've been told it's too early to find out baby's gender at a first trimester ultrasound, take a look a the nub theory: it's highly accurate from the 12th week of gestational age. Find out about the "angle of the dangle", and how to get the right ultrasound picture for seeing baby's gender at this stage.

What is a nub?

In early pregnancy, both boys and girls have a genital tubercle which will later become a boy's penis or a girl's clitoris. We call this the nub.

These pictures show a normal boy and girl in the first trimester (9 weeks gestational age); as you can see, they are almost identical. Amazingly, baby girls begin life with a pronounced phallus!

Through the 14th week of gestational age, male and female nubs are the same size.

Now you can see the problem in determining a baby's sex at this stage. It isn't because we can't zoom in close enough - it's because all babies look like a baby boy.

How does the nub theory work?

So, if boys and girls both have a genital tubercle at this point, how can we tell them apart? It's the angle of the dangle:

Boy nubs angle upward
Girl nubs lie down flatter

Note that isn't a theory; it's a known biological fact, and is due to the genital tubercle's development into a penis or a clitoris.

The nub angle can be seen in a profile image of the baby. Look for the angle of the nub compared to the baby's lower spine.

This side view is called the "sagittal plane" (pronounced SA-ji-tul), and so the angle of the genital tubercle in this view has been termed the "sagittal sign".

Male Sagittal Sign		Female Sagittal Sign

1. Draw the baseline.

This is the lower line you see in the images, and it's drawn along the base of baby's spine. Technically, the line is "through the lumbrosacral skin surface." Lubrosacral means the lowest part of the back down to just above the tailbone, so this refers to the area just above baby's bottom.

2. Draw the nub line.

Of course, the key to drawing the nub line is being able to identify the nub. You can practice by taking one of our quizzes on the nub theory, or view the ultrasound galleries for hundreds of examples, filtered by gender and week of pregnancy.

3. Determine the nub angle.

Boy = 30° or greater
Girl = 10° or less
Inconclusive = 10° to 30°

In the diagram, the horizontal line (0°) is the baseline (baby's lower spine). A nub with a pronounced lift is in the boy range; if it's barely raised, level, or aiming downward, it's in the girl range.

If it's in between -- in the grey area -- then the nub theory doesn't predict baby's sex.