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What is the Nub Theory?

the nub theory, what is it


You have had your dating scan at around 12 weeks gestation, your friends have told you about the angle of the dangle and you found your way to our website? Congratulations, you’re in safe hands! The Nub Theory is the study of the position of the genital tubercle, affectionately known as ‘the nub’.  The genital tubercle is the collection of dense tissue that will eventually become your baby’s genitalia. The nub is either the penis and scrotum, or clitoris and labia, and its angle in relation to your baby’s spine can tell us if you are growing a gorgeous girl or a beautiful boy.

Sometimes called the angle of the dangle, the nub is commonly agreed to sit below an angle of thirty degrees if baby is female, and above thirty degrees if baby is male- though this is somewhat open to interpretation- and we are confident that with our years of experience, our interpretation is the most accurate on the internet.

We look at so much more than the angle. We look at shape, size, length, relation to baby’s pelvic structure and other organs. The development of the nub to genitalia happens between around week ten, when the nub becomes visible, and week fourteen when distinctive genitals are formed.  The accuracy of the nub theory increases during this period. Though at ten weeks the nub will always appear female (below thirty degrees), there is only a 50% chance of it staying that way, in fact, the same could be said through the majority of the eleventh week; though some male foetuses can be identified in the latter half of the eleventh week, nubs that appear female still have a 50% chance of changing.  Yet by week 13, there is as much as a 95% chance of accuracy.

So, what happens in the twelfth week? Well, the labioscrotal folds previously contained within the genital tubercle are separating. In male foetuses the scrotum (but not testes) is descending below the penile bump, which is in turn being pushed up and away from the rump. In females, the labia are forming around and below the clitoral protrusion, fixing it into place.  Of course, these changes don’t mean that by the end of week twelve, baby has a fully developed set of genitalia. Both clitoris and penis will still protrude between the legs at a similar length, meaning sexing shots, or “potty shots” are still inaccurate at this gestation.

Studies have shown that during the twelfth week of gestation there is around an 85% chance of accuracy. However, these studies were based on live sonography, when the nub could be viewed from many different angles, and the position of the spine could be accurately determined. We believe that from still images displaying the nub and spine clearly, that before twelve weeks and four days the accuracy is closer to 75-80% and between twelve weeks and four days and twelve weeks and six days, the accuracy increases to 80-85%.

This is why submitting the exact gestation is really important. Nub theory is based on what baby measured on the day of your scan. Sometimes printed on the images, the measurement we are looking for is the gestation based on CRL (crown to rump length), normally printed as, for example; 12w 1d or 13w 4d (weeks and days).


Here at Babyboo Club we pride ourselves on our Nub Theory knowledge and have read indisputable evidence about its reliability. However; we will take this opportunity to state we are not doctors, and while the theory is credible and reliable, the studies are based on live images exclusively looking for the genital tubercle during a scan. Still images can be less accurate. We do not guarantee the gender with our predictions.