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Discover your baby’s gender from the start !

How Nub Theory can predict baby gender from 12 weeks scan

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What is the nub theory and is it reliable ?

The nub theory is a method used by professionals to get an early prediction of the gender of a baby from an ultrasound scan.

Nub theory is based on the analysis of the "nub" (also called genital tubercle), your child's indeterminate sexual organ. The nub will later turns into a penis in male babies and a clitoris in female babies. You can see this during the second trimester of pregnancy (on the anatomy scan around 20 weeks) but in early pregnancy, the nub is very similar for both girls and boys. They have the same look, same shape and same size.

Genital nub example on a fetal ultrasound

The first difference you will start to notice on a scan is that boys nub and girls nub will point out in different directions. Boy nubs angle upward while girl nubs lie down flatter. This is why nub theory look at what is widely called the "angle of the dangle" to guess if a fetus is a boy or a girl.

The nub theory consists of analyzing the angle between the baby spine (the lower line in the images below) and the nub (the upper line). If the nub forms a 30 degree angle or more in relation to the spine, it is likely a male fetus. If it is pointing straight out, under 30 degrees, or down, it is likely a female fetus.

The nub theory applied to a girl nub
The nub theory applied to a boy nub

Is the nub theory a reliable method for determining gender ?

The nub theory has been around since 1989. After that, many scientific studies have evaluated the accuracy of its predictions (in 1999, in 2006 and in 2012).

The method has an average reliability of more than 90% if:

Yet, the method is significantly less reliable if the ultrasound is performed before 12 weeks of pregnancy or if the nub is not clearly visible.

The accuracy rate will also highly depends on the ability and experience of the professionnal. One ultrasound technician explained to us that she got the gender wrong only once in a 25-years career (unfortunately this was for her best friend's baby !). But other admitted that they often don't even dare to reveal their prediction because they are not confident enough.

This is why the experts that work for Babygram are selected after a test that check their ability to provide reliable predictions.

Sometimes, the nub theory can be wrong

As explained, the nub theory is reliable only if we can see the baby from the side and if the nub is fully visible. However, babies are not robots: they move a lot in utero and very often the nub happens to be totally or partially hidden. The analysis can then be very difficult or even misleading, as shown by the cases presented below.

Partial view of a boy's genital nub
Partial view of a girl's genital nub

Sometimes the analysis is difficult to the point that one baby shown in two positions may present a very different looking nub: on the first picture below the expert’s eye sees a boy whereas on the second picture it looks like a girl ! Same baby, two nubs ! Likewise, two babies, girl and boy, can have a nub of the same appearance ! See the examples below which are great classics of cases that are very difficult to identify.

Nub theory can be wrong: different-looking nubs for the same baby girl
Limits of the nub theory: same-looking nubs but different genders

What’s more, sometimes the nub cannot be seen at all or its appearance is of no particular gender, as shown in the photos below. Boy or girl ? Predicting the sex in thoses cases is a difficult guess !

Limits of the nub theory: unclear examples of baby nubs

Finally, before 12 weeks of pregnancy and especially for fetuses smaller than 60mm, a flat nub indicating a girl is not reliable as the nub can rise and finally show a boy!

How BABYGRAM provides the most reliable prediction you can find online

When it is reliable, experts at BABYGRAM use the nub theory to analyze your ultrasound and provide a result up to 99% accurate.

In cases where the nub cannot be seen or interpreted, BABYGRAM uses its unique technology to compare your ultrasound to more than 10,000 images to deduce the gender of your baby with an average reliability of 85% from 9 weeks of pregnancy. This technology is based on an algorithm of artificial intelligence that is currently being presented to scientific journals for peer review and future publication.

Discover our gender prediction methodology in detail by looking at our article or order your ultrasound analysis now for €14.90 !


Get now our FREE ULTIMATE GUIDE to master the Nub Theory

preview of the Ultimate guide to master the nub theory

What people say about us

Linda T.

Great service, had my answer in 2 hours, and you got it RIGHT ! First baby girl after 2 boys !! thanks !

Amber B.

Confirmed Girl and you knew it even if I was only 9 weeks pregnant. Congrats to your team !

Emily J.

Happy to finaly find a serious gender prediction service online ! I was about to order Ramzi theory but I read your article explaining it was all fake ! THANK YOU

Jennifer and Michael

So excited to get a glimpse on our baby's gender, we were really impatient to know and our obgyn wouldnt give us a clue, a huge thank you !

Nadia H.

Top service, answer received in 3 hours apparently it would be a little girl it's also what my doctor told me but I wanted to confirm too happy with this second opinion, I will keep you informed at the time of the scan of the 2nd trimester!

Ashley MB.

A little boy for me, CONFIRMED at second scan ! Thanks for this service !

Mary F

You told me GIRL with 87% chance and your were right! A big Thank You to Stephanie from customer service she is so nice !

Patricia G.

At first I came here to use the chinese calendar predictor but you looked serious so I ordered an analysis. Result: confirmed boy at birth !

Carole K.

I am so happy with this service, I knew my grandchild's gender before my son and his wife who still dont want to know !

Lola S.

You got him right at 8 weeks only, thank you !


I got a bit confused when uploading the pics but apart from that I recommend your service!


Got my result in an hour with the express delivery, thank you !


At last I got a girl, I am thrilled thank you !!

June & Jon

You got her right ♥

Chiara D

There was no nub visible on my photo so your service was the only way to know my baby's gender and YOU GOT HIM RIGHT !! This is impressive, I wish you good luck to develop this amazing service !

Violetta R

My husband was skeptical but you were right on my baby girl despite all other guesses, thank you !!

Dora L.

It did not work for me so I was refunded, a bit disappointed but I appreciate your honesty.

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Our Story

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