Nub Theory: what it is and how much we can trust

It is generally thanks to an ultrasound at the 20th week of pregnancy that it is possible to see the external genitalia of the fetus and therefore know its sex.

However, browsing the internet you will find people who claim to be able to give you, with good probability, the same answer already at 12 weeks, provided that you have the image of an ultrasound done that week.

But what, exactly, is Nub Theory, and when is it truly reliable to predict the sex of your baby?

The Nub Theory, explained

Nub Theory revolves around the genital tubercle, which forms in early pregnancy on the baby’s lower abdomen. The genital tubercle is nothing more than a sketch of the genitals. In the end this ;core;, hence the English term ;nub theory; (hazel theory) turns into a penis in male children and a clitoris in females.

The idea behind Nub Theory is that if you can take a look at the genital tubercle, and in particular at the angle of the genital tubercle relative to the spinal cord, you can probably anticipate what will be evident with ultrasound in the coming weeks, namely the sex of the future unborn child.

In particular, according to this theory,

  1. The genital tubercle must be evaluated on the middle sagittal plane, that is, the plane that divides the body into the left and right parts and that lies on the midline. To do this it is necessary to have an ultrasound in which the length of the spine is visible in full, that is, the spine is parallel to the plane of the image.
  2. Ultrasound in the hand, you need to look for a bump, a small protrusion, in the place where the baby’s legs will form. This is the genital tubercle. If your baby’s bump is angled more than 30 degrees relative to his spine, this, according to this theory, indicates that the fetus is a boy. Conversely, if the angle is less than 30 degrees, the fetus is female.

What science says

The Nub Theory allows, at best, to increase the chances of guessing the sex of the child compared to guessing.

There are empirical studies that show that this probability is quite high.

But there are some factors that, in fact, make it quite difficult to apply it in reality:

  • To make a good reading of the angle the fetus must be placed on the side
  • the fetus must not have closed legs
  • the mother must have a low BMI body mass index: less than 24.

And, even if all the above conditions are met, it remains to be said that it may also happen that the child is developing slightly slower than expected and, if so, that the tubercle has not yet begun to point upwards. Or it may be that, from the ultrasound image, the umbilical cord is mistaken for a penis.

In short, the application of this theory certainly does not lead to extremely accurate predictions. It can be used as a diversion, but certainly not to make important decisions, as could be the choice of colors in the bedroom.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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