If you’re pregnant, chances are you’ve stumbled across the phrase nub theory on parenting forums, social media or even in conversations with other mums-to-be. It’s often mentioned as a fun way to predict your baby’s gender long before the traditional 20-week anatomy scan. But what exactly is it, and is there any science behind it?

What is the Nub Theory?

The nub theory is based on looking at the genital tubercle, sometimes called the “nub”,  which is present in all developing babies between around 11 and 14 weeks of pregnancy. This small bump eventually develops into either a penis or a clitoris.

The theory suggests that the angle of the nub in relation to the baby’s spine can predict whether the baby will be male or female:

• If the nub points upwards at an angle greater than 30 degrees, it may indicate a boy.
  
  

• If the nub is parallel with the spine (or pointing down slightly), it may indicate a girl.
  
  

How Accurate Is It?

At best, nub theory predictions are considered an educated guess. Some research suggests accuracy rates can be reasonably high after 12 weeks, but it is far from 100%. Several factors can affect accuracy, such as:

• The baby’s position during the ultrasound.
  
  

• The clarity of the image.
  
  

• The gestational age (before 12 weeks the nub may not have developed enough).

For this reason, medical professionals don’t use nub theory as a diagnostic tool. It’s more of a light-hearted prediction method that parents often enjoy while waiting for their official scan.

When Can You Find Out Your Baby’s Gender for Certain?

In Australia, the most reliable way to determine your baby’s gender is at the mid-pregnancy anatomy scan around 20 weeks. At this point, ultrasound can clearly visualise the baby’s genitals in most cases. Some parents may also find out earlier through non-invasive prenatal testing (NIPT), which can detect Y chromosomes from around 10 weeks.

Should You Rely on Nub Theory?

Think of nub theory as a bit of fun rather than a guarantee. If you’re curious, you can certainly ask your sonographer whether they can see the nub during your first trimester ultrasound, just keep in mind that their priority is to check your baby’s health and development.

Want to Know Sooner? Try a Gender Scan

If you’d like a more reliable answer before the 20-week scan, Ultrasound Insight offers dedicated gender scans from 15 weeks. These are non-medical scans (no referral needed) designed to give parents an earlier glimpse into whether they’re expecting a boy or girl. 

They’re a popular choice for families who want to share the news with loved ones or plan a gender reveal with confidence.

At Ultrasound Insight, our focus is always on safe, evidence-based care. While we’re happy to explain what you’re seeing on screen, we recommend waiting for your 20-week scan or medical testing for accurate results.

Bottom line: Nub theory can be entertaining, but it’s not a substitute for professional medical advice or diagnostic imaging.