Nub Theory Explained: Predicting Your Baby's Gender

Pregnant woman sitting on her bed with an ultrasound image and white baby socks

Whether you believe them or not, old wives tales are a fun way to guess the gender of your baby. And there are so many out there to test and try.

You can dangle a wedding ring, have your tea leaves read to you or guess based on your food cravings, along with so many more. But one theory that keeps popping up is the nub theory.

If you’re thinking “what is the nub theory?” and “what is a nub?”, you aren’t alone. Below we explain the nub theory and if it’s a reliable way to discover your baby’s gender.

What is the nub theory for gender?

The nub theory believes that the genital nub on the baby can determine the gender before the external genitals are even fully formed.

The external genitals of a fetus are developed at around week 4 of your pregnancy. But boy or girl genitalia remain nearly identical until around 18 to 20 weeks.

However, according to the nub theory, you won’t need to wait until week 20 to learn the gender. You could discover the gender as early as the end of the first trimester.

How do you work out the nub theory?

So how do you understand the nub of a baby? And what is a nub?

A nub is a protruding mass of tissues, it’s hardly noticeable, but it starts developing at around 4 weeks. The boy and girl nubs are exactly the same until around the 14-week mark. Before then, you won’t be learning anything new.

You need a clear profile of the baby at the end of the first trimester. To perform the nub test, you’ll measure the angle of the baby’s nub in relation to the line of the baby’s spine. Those who love and live by the nub theory refer to it as “the angle of the dangle”.

A baby girl has an angle below 10 degrees, meaning the nub runs parallel to the spine or bends slightly towards it.


nub theory angle on a female fetus

(Photos from Healthline)

A baby boy will have a dangle that is 30 degrees or higher. It can be a little tricky to determine if it's precisely 30 degrees, so if the nub is pointing definitively upward and away from the spine, you can guess it’s a boy.


nub theory explained on a male fetus

(Photos from Healthline)

Unfortunately, if the nub is between 10 and 30 degrees, you won’t be able to use the nub theory to determine the gender.

Is the Nub Theory Correct?

The accuracy of the nub theory depends on what corner of the internet you’re reading.

Some parents say it was absolutely correct and some parents say it's merely not true.

Several studies have been undertaken to determine just how accurate the nub theory is. The data does vary slightly per study, but the key takeaway from all of them is that the earlier the ultrasound, the less accurate the results.

According to a study by the Australian Journal of Ultrasound in Medicine, gender prediction shot up in accuracy for fetuses past 14 weeks.

This study had 54% gender prediction accuracy before 12 weeks, but this figure went up to 100% accuracy (according to the study) after 14 weeks.

The Journal of Medical Ultrasound conducted a study and found that at a gestational age of fewer than 12 weeks, gender prediction accuracy was significantly lower than after the 12-week mark. They also found that the accuracy increased when the ultrasound technicians had more experience, after performing 200 ultrasounds, the accuracy rate was around 71.4-78%.

And finally, a study in the Czech Republic researched how feasible it is to see and assess the nub’s angle. The study found that the most common issue was the nub falling between the 10 and 30-degree windows, thus being excluded from the nub theory.

The feasibility to see the nub clearly under 11.5 weeks was only 39.5%, after 12 weeks; it was still low at around 63%.

Pregnant woman laying on her bed with an image of an ultrasound

Why You Should Approach This Theory With Caution

Some uncontrollable factors may affect the accuracy of the nub theory in your particular pregnancy.

1. The Amount of Experience Your Ultrasound Technician Has

The more ultrasound reading your technician has under their belt, the higher their accuracy rate is. If the trained scientists in these studies find it tricky to identify the nub correctly, it’s likely your attempts will be less statistically valid.

2. The Position of Your Baby

The Nub theory will only work if your baby is perfectly angled during a clear ultrasound. These scanning conditions are optimal but are rare. Babies love to wiggle and move, and sometimes it's just tricky to have perfect image clarity.

3. Weeks 11 and 14 Differ Hugely

End of first-trimester ultrasounds typically falls between weeks 11 and 14. However, in terms of fetal development, there is a considerable discrepancy between weeks 11 and 14. If your scan is during the phase when the Nub theory is often incorrect, you won’t guess the gender easily.

There Are Better Ways to Find Out the Gender

The easiest way to learn the gender is to wait until weeks 18 to 20 and have an ultrasound technician make an educated prediction.

If you are interested in knowing earlier, specific blood tests can tell you the gender and other genetic information.

And if you want the gender to remain a surprise but you’re still a little curious, why not research all of the old wives tales and make an average? They’re fun to play with and a fun way to bond with friends and family.

In Summary

Unfortunately, the nub theory isn’t the most accurate way to predict your baby’s biological gender. However its a lot more fun the make a more educated guess at the sex, and it’s likely to be more accurate than other old wives tales out there.

We recommend you get a prediction from your ultrasound technician or a blood test before you commit to a decorating theme for your new baby.

Gender aside, connect with your baby from the comfort of your own home with our fetal dopplers. Our dopplers have a variety of features to suit your every need, shop now!


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