Nub Theory: What Is It And How Accurate Is It

Nub Theory

Image: iStock

If we told you that you could know the baby’s gender through the first-trimester ultrasound, would you believe? It is entirely possible with the Nub Theory, an accurate gender prediction method relied upon even by experts. In this post, MomJunction helps you understand all about this theory and how best it works.

What Is Nub Theory?

During the early weeks of pregnancy, i.e., from 11 to 13 weeks, babies are known to have a nub called genital tubercle between their legs (1). It is otherwise referred to as the angle of the dangle, which indicates if it is a girl or a boy. If the angle of the nub is over 30° from the spine, it is a boy. If the angle is below 30°, it is a girl (2).

The angle of the nub is determined by carefully examining the ultrasound scan picture.

Back to top

How Does Nub Theory Work?

An ultrasound scan is necessary to determine the sex of your baby using the nub theory in the first trimester. The nub angle is visible in the profile view of the baby. You will have to compare the angle of the nub to the baby’s lower spine.

The side view or profile of the baby in an ultrasound image is known as the sagittal plane. Hence, the angle of the nub is referred to as the sagittal sign, which is used to determine the gender of the baby. Here’s how (2):

1. Draw the baseline:

Consider the sagittal image of the scan and determine the baby’s base spine. This is the lower line you can see in the image and is technically called the lumbosacral skin surface. It is the lowest part of the back just above the tailbone. This becomes the reference line in the nub theory.

2. Draw the nub line:

Once you mark the baseline, find the nub. Ultrasound images are not always clear, and a little bit of practice reading the ultrasound images of the same pregnancy week can help in identifying and locating the nub. Once you find it, draw a line across it.

3. Calculate the angle:

Now that there are two lines in the scan image, you should determine the angle between them. If the angle is greater than 30°, you can say it is a boy. And if the angle is less than 10°, or the nub line is parallel to the baseline, you can say it is a girl. If the angle is between 10 and 30°, the result is inconclusive. In that case, you can try to know the gender again by getting another scan after a couple of days.

Back to top

[ Read: Ring On A String Gender Prediction ]

How Do Gender Experts Predict Gender Using Nub Theory?

Gender experts depend on the visibility of the genital tubercle to predict the sex of your baby using the nub theory. This visibility can be affected by the position of your baby and their movements.

The amount of amniotic fluid, the thickness of the abdominal wall, and the scan quality can also determine the results.

The following are the criteria for predicting the sex of a baby using the Nub Theory:

  • Visibility
  • The nub’s angle to the baby’s spine
  • The shadows on and around the nub captured in the scan
  • The length and shape of the baby as per the gestational age

Before the tenth week of pregnancy, the genital tubercle looks alike in boy and girl fetuses. At 11 weeks, the view is clearer and may allow you to predict the gender of your baby under perfect conditions. The accuracy of nub theory is the highest in or after the 12th week of gestation.

Back to top

How Accurate Is The Nub Theory Of Gender Prediction?

While most people are skeptical, Nub Theory is a scientific method of gender determination. Unlike the old wives’ tales, this theory doesn’t consider illogical explanations to find out the sex of your baby. However, the accuracy of the method depends on the timing of your ultrasound (3).

11th week:

  • Though the sex organs of the baby begin to develop internally by the ninth week, they are not visible externally. Therefore, the accuracy is poor this week.
  • In almost 50% of the cases, the nub angle is between 10 and 30°, and gender prediction is impossible.
  • Even if the angle is visible, it may change in the following weeks.
  • Between indeterminate and inaccurate nubs, the chance of obtaining an accurate rate in this week is only 67%.

If the results are inconclusive, wait for a week and get a scan again.

[ Read: Scientific Methods To Predict Gender ]

12th week:

  • The external genitals of the baby are clear and inclined towards one gender.
  • About 7% of nubs are indeterminate, and they can go either way of the range.
  • The chance of accuracy is 98%. It means one in 50 babies predicted to be a boy will turn to be a girl.

13th week:

  • The baby’s nub turns to be more real, and there is little chance of the prediction going wrong.
  • However, about 6% nubs are still indeterminate.
  • The accuracy rate is 100% if you use the theory right.

The combined results from several studies are as follows (4) –

Week Of GestationGender Accuracy (%)Total Accuracy (%)


Back to top

Why Is The Nub Theory Gender Determination Medically Important?

Some of the sex-linked disorders affect only the male offspring. Therefore, determining the sex in the early stages of pregnancy can allow parents to go for counseling if the fetus is suspected to have any genetic disorders.

If the nub theory predicts a baby girl, it requires no further invasive testing such as amniocentesis or CVS to find if the baby is healthy. On the other hand, if the nub theory determines a boy, further tests are required to evaluate the health concerns of the unborn.

[ Read: Chinese Gender Prediction Test ]

Back to top

How To Get The Perfect Nub Shot – Tips For The First Trimester Ultrasound:

Gender prediction through Nub Theory largely depends on the picture of the ultrasound. The nub shot has to be perfect for you to determine the result. So here is how you can improve the chances of a good picture:

1. Schedule the scan at 12th week or later.

Nub theory can give you accurate results only after the 12th week of pregnancy. The accuracy of this theory depends on the gestational age determined by the baby’s crown-rump length (CRL) (5) and is higher after the 13th week.

Pregnancy weekCRL
11th week45 – 54.9mm
12thweek55 – 69.9mm
13thweek70.0 – 82.4mm

If your pregnancy weight is on the higher side, wait for one more week. The thick abdominal tissue can obscure the image. Moreover, you will have a greater chance for better pictures if the baby is slightly larger.

2. Get the sagittal plane view of the baby.

This is the profile view of the baby, with a straight spine. This not only helps you view the nub but also lets you capture the angle of the dangle correctly. The ‘potty shot’, the exact moment in the ultrasound when the baby’s gender can be known, is used for gender determination only in the second trimester (6).

3. Practice finding the nub.

Sometimes it is easy to find the nub, and sometimes it is not. You can practice by checking different ultrasound pictures and taking quizzes.

[ Read: Symptoms Of Baby Boy During Pregnancy ]

4. Drink more water before the ultrasound.

The more amniotic fluid you have, the clearer your ultrasound image will be. So drink ample water a week or two before you are due for an ultrasound. This can help increase the volume of the fluid in your womb.

5. Consume apple/orange juices 30 minutes before the scan.

To capture the baby right, you may want your baby to be a little active during the ultrasound. This will ensure that your baby moves a little, and the sonographer can capture the right nub shot. Have a glass of apple or orange juice about 30 minutes before the scan. Anecdotal evidence claims this can make babies more active in utero.

Often, people confuse the Nub theory with the skull theory of gender prediction.

Back to top

Is There Any Difference Between Nub Theory and Skull Theory?

Skull Theory is different from Nub Theory. This method looks for certain male and female markers in the shape of the skull to identify the gender. The baby’s 12th-week scan picture, magnifying lens, and knowledge about the shapes of female and male skulls are needed to predict the gender.

For Nub Theory gender prediction to work, the baby has to be in the right position for the sonographer to capture the pictures. If your baby is turning away or kicking wildly, you may not get to see the actual angle.

So if you want an early gender prediction, hope that your baby stays in position for the picture, and your Nub Theory prediction is as accurate as it can get after the 12th week of pregnancy.

[ Read: Symptoms Of Baby Girl During Pregnancy ]

Have you had success with the Nub theory gender prediction method? Share your story in the below comment section.

Recommended Articles:

Was this information helpful?

Comments are moderated by MomJunction editorial team to remove any personal, abusive, promotional, provocative or irrelevant observations. We may also remove the hyperlinks within comments.
The following two tabs change content below.

Rebecca Malachi

She is a Biotechnologist with a proficiency in areas of genetics, immunology, microbiology, bio-engineering, chemical engineering, medicine, pharmaceuticals to name a few. Her expertise in these fields has greatly assisted her in writing medical and life science articles. With 8+ years of work experience in writing for health and wellness, she is now a full-time contributor for She is passionate about giving research-based information to readers in need. Apart from writing, she is a foodie, loves travel, fond of gospel music and enjoys observing nature in silence. Know more about her at:
FaceBook Pinterest Twitter Featured Image