The Nub Theory

Before 15 weeks gestation both male and females have genital tubercles which appear to be identical, however there is a notable difference to the position of the nub often referred to as the ‘Angle of The Dangle’. A male foetal protrusion points approximately 30 degrees up relative to the backbone whilst a girls’ protrusion is below 30 degrees.

Numerous studies published in medical journals have assessed thousands of pregnancies to determine that the angle of the genital tubercle can predict gender in the first trimester with accuracy increasing from gestation weeks twelve to fifteen. Ultrasound scans between 12-15 weeks with a profile view can be analysed by the angle and shape of the nub.

The two images below are profile shots of the scan and at this age of gestation the best view of the nub, Therefore when submitting your image ensure the scan is a profile image.


Image 1: Male fetus in early development. With the male fetus, the genital tubercle usually creates an angle of greater than 30° with the lower part of the spine. Baby Gender was identified by the 14 week ultrasound scan

Confirmed boy nub

Image 2: Female fetus in early development, the genital tubercle protrudes in the same direction as the lower portion of the spine with an angle of less than 30° relative to the backbone. Gender determination identified at 12 week ultrasound scan.

Confirmed Girl nub

How Accurate Is The Nub Theory?

The nub theory has increased accuracy the longer the gestation. A scan at eleven weeks may not reveal anything, at twelve weeks gestation the nub theory is approximately 75% accurate and just one week later at thirteen weeks it’s thought to be around 94% accurate*.

Gender determination accuracy by weeks.

12 Weeks 88%

13 Weeks 94%

14 Weeks 98%

15 Weeks 99.6%

The Nub theory is backed up by various medical studies which can be found within our research area.

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*- Sourced for the following study conducted by:

Efrat, Z., Perri, T., Ramati, E., Tugendreich, D. and Meizner, I. (2006), Fetal gender assignment by first-trimester ultrasound. Ultrasound Obstet Gynecol, 27: 619–621. doi:10.1002/uog.2674