The Nub Theory

What is the Nub Theory

Reaching the second trimester is a big milestone for any pregnancy. At this stage almost all parents have had at least one ultrasound of their baby. It is also at this stage when the nuchal translucency test is performed via ultrasound. Although some parents might have done a clinical blood test by this 12th week of pregnancy to find any chromosomal abnormalities, most will still not know the gender of their baby.

The ultrasonographic determination of fetal sex in the second trimester is fundamentally based on the demonstration of the penis and scrotum in the male fetus and of the labia majora and labia in the female fetus. However, at earlier gestational ages these signs are not useful as there is no significant difference in the size of the penis or clitoris before week 14.

The nub theory 12 week ultrasound is an great tool for many soon-to-be parents to see the defined gender markers of their baby before the genitalia is fully formed.

Advances in ultrasound technology in recent years have led us to be able to accurately predict the sex of the baby by analyzing sonograms from 10 weeks onwards based on the direction in which the genital tubercle points in relation to the spine. Thanks to the high quality of modern diagnostic imaging, we are able to see the genital tubercle develops into a penis or if it will develop into labia and clitoris.

Nub Theory accuracy

Medical studies published in Ultrasound Obstet Gynecol. 1999 May;13(5):305-7 as “First-trimester determination of fetal gender by ultrasound” record the controlled study that after 12 weeks of age the genital tubercle start showing very reliable gender markers for both male and female. The official consensus of nub theory is that both male and female genital tubercles look the same until 12 weeks. After this pregnancy milestone, male nubs will raise over 30 degrees in comparison to the angle of the spine. Female nubs on the other hand will remain almost parallel to the spine. Starting at 12 weeks nub theory is over 90% accurate, reaching up to 99% accuracy at 14 weeks.

There are several factors that can affect the correct identification of the baby’s sex when applying the nub theory. The factor that most affects is the resolution with which the ultrasound is taken, as mentioned above the main reason we are able to make these early predictions is high quality of the images. In close second is the position of the baby, since it is often impossible to see the genital tubercle if it has crossed legs or if the baby is facing away from the imaging. For an accurate nub theory prediction the baby must present itself in full profile view, especially as it pertains to the lower half of the body.

Likewise, the progressive maturation of the male genitalia responsible for the erection of the penis in the male, together with the development of the scrotum, forces the penis to have a vertical position and helps us to give a more precise diagnosis in the prediction of sex. In many instances, when applying nub theory at 12 weeks ultrasounds, our staff is able to see the stacking above the nub, which pertains to penis tissue, and the bulging under it which pertains to the developing scrotum. Both of these extra pieces of information can add to the accuracy of a prediction, or even help our professionals confidently make one when the angle of the nub is not as visible as we recommend. In the same way, when looking at a female nub after 12 weeks of gestation, in many cases the start of a developing clitoris can be seen above the nub. For the untrained eye this can be very easily confused with a stacking of the developing penis.

How to distinguish the nub in your own ultrasound?

Ironically enough, the most difficult part of trying to read your own ultrasound is finding the nub itself. For this, it is imperative that your ultrasound has been taken at 12 week or beyond. We understand that every baby develops at a different rate, and sometimes at 11+ weeks a prediction can be made, however the overall rule is after 12 weeks. Secondly ensure that your 12 week ultrasound shows the baby from a lateral or profile view and therefore the angle or protuberance is known as a sagittal sign. Once all these requirements have been met, you will have to find the spine of the baby which will show from the neck all the way down to the rump. Right above the spine, about half way the way to the umbilical cord you will be able to see a small line that is more external than the rest, and that is the genital tubercle.

A common difficulty during this process is that there are a lot of markings that are bones, tissue and even the umbilical cord can be in the way. This is why we only recommend nub theory predictions to be taken as accurate if a professional in the field has provided them.

Once the nub has been located, the next step is to know if it is angled in relationship to the spine. The angle of the spine should be taken carefully, considering if the baby is sitting of if the lines need to be adjusted due to the view of the baby or the movement it was performing at the time of the ultrasound. It can be difficult to appreciate and quantify the angle of inclination since sometimes the male nub between weeks 11-12 may not have yet risen, in these instances it is recommended to look for any other gender signs such as the clitoris, developing penis or scrotum.

The angle of the sagittal sign is a good marker to predict the baby’s self but it is not the only one. There are other markers in the tuber area that can indicate the sex of the baby and only the eyes of our experts can locate them. In addition, for the most accurate gender prediction we recommend combining your findings with Ramzi and Skull theory.