Baby Gesture

Understanding the Nub Theory at 13 Weeks of Pregnancy

March 7, 2024 | by Carlene Kuntz

a man standing on


During pregnancy, parents eagerly await the moment they can find out the gender of their baby. While many choose to wait until the 20-week ultrasound, there is a theory that claims to predict the baby’s gender as early as 13 weeks. This theory is known as the Nub Theory.

What is the Nub Theory?

The Nub Theory is based on the idea that around 11-13 weeks of pregnancy, a developing baby will have a genital tubercle. This tubercle can potentially indicate the baby’s gender, although it is not always accurate. The theory suggests that the angle of the tubercle can be an indicator of whether the baby is a boy or a girl.

How Does the Nub Theory Work?

At around 13 weeks, the genital tubercle will start to develop into either a penis or a clitoris. The Nub Theory focuses on the angle at which this tubercle is positioned in relation to the baby’s spine. If the angle is greater than 30 degrees, it is believed to suggest a boy. If the angle is less than 30 degrees, it is thought to indicate a girl.

It is important to note that the accuracy of the Nub Theory depends on several factors, including the clarity of the ultrasound image, the position of the baby, and the experience of the sonographer. Therefore, it is not a foolproof method for determining the baby’s gender.

How to Determine the Nub Angle

To determine the nub angle, you will need a clear ultrasound image of your baby’s profile. The sonographer or ultrasound technician will need to capture an image that shows the baby’s spine and the genital area. Once you have this image, follow these steps:

  1. Identify the baby’s spine, which will appear as a straight line.
  2. Locate the genital tubercle, which will be positioned near the baby’s legs.
  3. Measure the angle between the baby’s spine and the genital tubercle.

Remember that accuracy is crucial, so it is best to have a trained professional perform the measurements.

Accuracy of the Nub Theory

While the Nub Theory can be exciting for parents looking for early gender clues, it is important to manage expectations. The accuracy of the theory varies, and it is not considered a definitive method for determining the baby’s gender.

Studies have shown that the Nub Theory has an accuracy rate of around 70-90% when performed by experienced sonographers. However, this accuracy decreases when the ultrasound image is unclear or the baby is in an unfavorable position.

It is also worth noting that the Nub Theory is more accurate in predicting male fetuses than female fetuses. This is because the angle of the genital tubercle is more distinct in males.

Other Methods for Determining Gender

If you are looking for more accurate methods to determine your baby’s gender, there are other options available. These include non-invasive prenatal testing (NIPT) and amniocentesis, which can provide definitive results. However, these tests are typically performed later in pregnancy and carry their own risks.

Alternatively, you can simply wait until your 20-week ultrasound, which is considered the most reliable method for determining the baby’s gender.


The Nub Theory can be an exciting way for parents to try and predict their baby’s gender as early as 13 weeks. While it is not always accurate, it can add an element of fun and anticipation to the pregnancy journey. However, it is important to remember that the Nub Theory should not be relied upon as a definitive method for determining the baby’s gender. For more accurate results, consult with a healthcare professional or wait for the 20-week ultrasound.


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