The Nub Theory

The nub theory is the theory used to decide whether you are having a boy or a girl. Its the ‘angle of the dangle’. The genital tubercle develops into either a penis or clitoris and this is possible to view from around 12 weeks by analysing the ‘nub’ in relation to the spine and other factors such as nub shape.

The following image is a perfect boy nub shot at 14 weeks gestation.


Basics of the Nub Theory

The male baby will have a nub that angles upwards at 30° or more. The female baby will have nub that is angled lower than 30° relative to the spine. If your unsure on how to read your scan, then gender predictors can tell you with degrees of certainty, depending on your stage in the pregnancy, which sex your baby is. This is known as the Nub theory. Accuracy rating increases much nearer to week fifteen (99.6% males and 97.4% females).

Image 1: Below is a profile view of a baby ultrasound  This scan was later confirmed boy. The nub area has been marked to clearly identify a male nub.

boy nub

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.

Genital Tubercle Development

External gender development for both male and female start the same. Between gestation weeks 7-9 the ‘nub’ is visible and looks identical. The correct term for the ‘nub’ is genital tubercle. The genital tubercle comprises of three areas, the genital tuber, Urogenital fold and the Labioscrotal fold.

Fetal Gender Development

Boy Nub 

Between weeks 9-11 gestation the genital tuber develop into the glans of the penis, the urogenital fold becomes the shaft and the labioscrotal fold matures into the scrotum. Between gestation weeks 12-15 the male nub will look very different to the female and will be located immediately behind the umbilical cord. The testicles remain within the abdomen until approximately weeks 28-32 gestation.

Image 2: 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. The nub shape is a simple line with ‘stacking’  just above the blue line. Baby Gender was identified by a 16 week ultrasound scan.

Confirmed Male Nub. shape of the nub

Girl Nub

In females, the nub development is much more static over the coming weeks. The urogenital folds develop into the Labia minora and the genital tuber continues development into the clitoris. Ovaries are fully formed by gestation week 12.

Image 3: 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 later confirmed at a 15 week ultrasound scan.

Confirmed Girl Nub

Does the Shape of the Nub matter?

As the term ‘Angle of the Dangle’ suggests its all about the angle of the nub first and foremost. However..

We do study other factors such as length, Anogenital distance (AGD) where possible, and the shape of the nub. Gender Predictors have seen many ‘fork’ and ‘stacked’ nubs later confirmed for both boy and girl. The reason for this is mainly due to the still image of the ultrasound scan that is submitted. The Nub theory has been studied by many medical researchers who during their study used live images from the ultrasound examination and thus clarification of other factors are much clearer than what a still image can offer.

Is the Nub theory accurate? 

The accuracy of the nub theory increases the longer the gestation, therefore the older the gestation date the higher the accuracy.

Gender determination accuracy by weeks. *

12 Weeks 88%

13 Weeks 94%

14 Weeks 98%

15 Weeks 99.6%

Please Note: Before submitting your scan for the Nub theory. Please ensure the image is in profile view (as above) and your scan is dated between weeks 12-15. The Ramzi theory may be better suited If your scan is dated earlier than this and within the 6-8 gestation range.

Submit Your Scan

*- Sourced for the following research 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