Skip to content

Nub Theory: How Accurate Is The Nub Theory?

Table of Contents

The Nub Theory

Table of Contents

Gender reveal or the determination of the gender of a baby during pregnancy is one of the most exciting and anxiety-inducing things most parents experience. Multiple old-wives-tales and non-invasive methods like the Ramzi theory and the Nub theory are used by parents across the globe in the hope to determine the gender of the baby before birth.

Speaking particularly of the Nub theory in this blog, let’s understand what is the nub theory, how accurate it is, and everything else you’d ever need to know about it!

So, What is The Nub Theory?

Babies, during the early weeks of pregnancy and while in the womb of the mother, have a nub which is also called the genital tubercle between their legs. Also known as the ‘angle of the dangle,’ the angle at which the nub sits determines the sex of the little one. 

  • If the angle of the nub is over 30° from the spine of the baby, you are supposed to have a baby boy
  • If the angle, however, is below 30°, you are supposed to have a baby girl

But What is a Nub or The Genital Tubercle?

A nub of a fetus, in simple terms, is a very small part of the baby’s anatomy, yet to be developed into a gender-specific genital. All babies have genital tubercles before the 15th week of gestation and these genital tubercles are referred to as the ‘nub.’

How is The Angle of The Nub Determined In The Nub Theory?

The angle of the nub, while using nub theory to determine the gender of the baby, can be determined only by examining the ultrasound scan picture of the fetus. 

How Accurate is The Nub Theory in Sex Prediction?

The accuracy of the nub theory completely depends on when the scan or the ultrasound was taken; which implies that the prediction is only as good as the timing of the ultrasound.

Acta Obstetricia et Gynecologica Scandinavica analyzed the ultrasound scans taken between weeks 11 to 13+6 of 496 pregnant women in their first trimester to determine the accuracy of the nub theory. Interestingly, once all the 496 scans were combined, it was found that “sex determination accuracy vis the nub theory improved with increasing gestational age, starting from week 11 to 13+6.”

While the accuracy rate was just 71.9% at week 11 pleas, the rate increased to 92% at week 12 and hiked to a whopping 98.3% by week 13!

How to Use The Nub Theory to Predict The Gender of a Baby?

As mentioned, an ultrasound or a scan is the first step to being able to use the nub theory and determine the gender of the fetus. It is only when the nub angle can be clearly seen and compared to the baby’s lower spine that the nub theory can be used.

Before you now reach out for the scan and learn how to use the nub theory, it should be noted the side profile of the baby in an ultrasound image is known as the sagittal plane and hence, the angle of the nub is referred to as the sagittal sign. Follow the below-given steps to use the sagittal sign and predict the gender of your baby using the nub theory.

1. Draw A Baseline On The Scan

In step 1, you will need to draw a reference baseline on the scan to apply the nub theory. Using the sagittal image of the scan, first determine the base spine of the baby. You should be able to see a lower line in the image, at the lowest part of the back and just above the tailbone. Voila! You have the reference line to use the nub theory right there.

2. Draw The Nub Line

Now that the baseline is out of the way, it’s time to find the nub. It might take you some time to be able to spot it as scan images aren’t necessarily clear or definite images. Keep at it, look for it and when you spot it,  draw a line across it.

3. Let's Calculate The Angle!

Now that you have two definitive lines- the baseline and the nub line- try determining the angle between the lines; this can be accurately done using a protractor.

  • If the angle of the nub is over 30 degrees from the spine of the baby, you are supposed to have a baby boy
  • If the angle, however, is less than 30 degrees, you are supposed to have a baby girl
  • If the angle is between 10 and 30 degrees, you would need to wait for a couple of more days and take another scan to come to a conclusive prediction

How Does The Nub Theory Point to a Boy?

To apply nub theory to the ultrasound of your baby, you must capture them in a clear profile so that the length of their spine can be seen horizontally. You would then look for the nub, or a small protrusion, in the space in between where your baby’s legs will develop.

According to nub theorists, if your baby’s nub is angled more than 30 degrees about its spine, it is a boy. Although no one suggests using a protractor to determine the precise angle at this point, this is evidently where nub theory becomes somewhat muddled.

What exactly does a 30-degree ultrasound look like? We don’t know, but if you draw a straight line along the bottom half of your baby’s spine—roughly where their butt is—on the ultrasound, you can see if the nub is pointing up or down from that line.

If it is, it seems to be a boy.

Do Experts Use Nub Theory to Predict The Gender of a Baby?

Yes, multiple gender experts do use the nub theory to help parents determine a baby’s gender and they do so while heavily depending on the visibility of the genital tubercle. This visibility can be affected by the position of your baby and its movements.

Certain requirements of criteria need to be taken into account before using the nub theory to predict the gender of the baby as follows-

  • Visibility of the genital tubercle
  • The angle between the nub and the baby’s spine
  • Shadows on and around the tip of the nub 
  • Shape, length of the fetus as per the gestational age 

It must be made sure that the nub theory is not used to determine the sex of your baby at least before the 12th week of pregnancy. This is because, in terms of the nub theory, in particular, the genital tubercle of both the male and female fetus looks pretty much the same. The scans and the view becomes clearer by week 11 and can provide a more accurate prediction by 12 weeks of pregnancy.

As per the biomedical papers of the Medical Faculty of the University Palacky, the Czech Republic, “The accuracy of nub theory is at its highest possibility in or after the 12th week of sex prediction and gestation before 12 weeks is not encouraged.”

Medical Professionals on The Nub Theory

Yes, even medical professionals and doctors use the nub theory to determine the gender of the baby and it, apart from just helping determine the gender, also helps understand if the fetus is suspected of having any genetic disorders. Since some sex-linked disorders affect only baby boys, the nub theory comes in particularly handy to understanding potential disorders in the early stage of pregnancy.

Typically doctors do not recommend any tests if the nub theory can predict a baby girl, however, additional tests like amniocentesis or CVS may be advised to determine the health of the baby if the nub test predicts a boy.

How to Get The Best Nub Shot of My Baby In The Ultrasound Scan?

Since the nub theory heavily depends on the visibility of the genital tubercle or the nub to predict the gender of the baby it only becomes super important for you to get a good shot of the baby’s nub in the scan. Here are some steps you could take before your ultrasound to get a clear image of the fetus:

1. Hydrate With Water

Though you will anyway need to consume a lot of water to even be eligible for the ultrasound, it is recommended to drink as much as water possible, even if it means overdoing it, before taking a scan to use the nub theory. This is necessary because drinking more water can help increase the volume of fluid in the womb and the more amniotic fluid, the clearer the ultrasound image and the subsequent image.

2. Consume Orange/Apple Juices Before The Scan

Having an orange or apple juice is also further advised before a monograph as they can have the baby become more active and make them move inside the womb, oftentimes getting a better shot as a result.

Nub Theory v/s Skull Theory: What are The Differences?

The skull theory is nowhere similar to the nub theory. Unlike the nub theory for gender prediction, it analyses the shape of the skull to determine the baby’s sex.

Nub Theory of Gender PredictionSkull Theory of Gender Prediction
The Nub Theory for gender prediction is done by using the baby’s scan to determine the baby’s angle at which the nub sits. This helps determine the sex of the little one.  The scan should be taken at least after 11 weeks of pregnancy.The Skull Theory of gender prediction uses a magnifying lens, and some knowledge about the shapes of female and male skulls to predict the sex of the baby. They can and should be taken at least after 12 weeks of pregnancy.

Final Thoughts: How Accurate is The Nub Theory?

The nub theory, like most other theories, cannot be considered 100 percent accurate at all times. Parents need to remember to leave some room for flaws. For instance,  if the baby is kicking or turning away wildly, you might not be able to see the needed angle. In other cases, if the baby’s genitals are not fully developed, it may become difficult to be able to even identify the nub. However, still, these methods can always be tried for fun and to ease the anxiety at least a little bit!

Here are Some Better Methods for Predicting The Baby’s Gender

Before an anatomy scan The majority of pregnant women undergo a first-trimester screening that includes an ultrasound and blood tests for chromosomal abnormalities like Down syndrome and trisomy 13. This typically takes place between 11 and 14 weeks of pregnancy and includes the same ultrasound that advocates of nub theory assert can be used to determine the baby’s sex.

At this stage, prenatal blood tests typically look for protein and hormone levels that could indicate abnormalities in the fetus. However, if you are at risk for other abnormalities, particularly sex-linked disorders like Duchenne muscular dystrophy and hemophilia, your physician may recommend including a blood test that can identify the sex of a baby.

FAQs: How Accurate is The Nub Theory

1. How accurate is the nub and skull theory?

Much like the nub theory, the skull theory of gender prediction is also not 100% accurate and only offers a 75%- 80% accuracy or viability.

2. Can the nub theory be wrong?

Since the efficacy and accuracy of the nub theory are supposed to range between 80-92%, the results of the predictions of a nub theory can very well be wrong. It is so because multiple factors affect the accuracy of this theory concluding the visibility of the nub, the angle of the fetus, the thickness of the mother's abdominal wall, and even the amount of amniotic fluid present.

3. How to increase amniotic fluid before an ultrasound?

If you are planning to take an ultrasound scan before applying the num theory, it should be noted that more amniotic fluid = greater visibility of the fetus at your ultrasound. A mother should make sure to have a lot of water and/or juices like apples and oranges in order to increase the amount of amniotic fluid before a sonogram.

4. What other theories apart from the nub theory can be used to predict the gender of a baby?

The Ramzi theory, the Chinese birth calendar, and the skull theory are other popular theories to predict the gender of a baby. It should be noted that taking a gender reveal ultrasound is perhaps the best and the most accurate way of predicting the gender of a baby.


On behalf of the editorial team at Parenthoodbliss, we follow strict reporting guidelines and only use credible sources, along with peer-reviewed studies, academic research institutions, and highly respected health organizations. To learn about how we maintain content accurate and up-to-date by reading our medical review and editorial policy.

Share this Article

Disclaimer: All content found on our website is published for informational and/or educational purposes only; not intended to serve or offer any form of professional/competent advice. We put in every effort to ensure that all information is just, accurate, fool-proof, useful, and updated but do not assume responsibility or liability, to loss or risk, personal or otherwise, incurred as a consequence of information provided. Parenthoodbliss may earn commissions from affiliate links in the content.

Rectangle 22

Did not find what you were looking for?

Drop-in your request and we will be happy to write it down for you!