Nub theory: will a first-trimester scan show the sex of my baby?

It’s too early to find out the sex of your baby at your dating scan or nuchal translucency (NT) scan.

The kind of scan machine your sonographer uses isn't high-tech enough to easily show the physical difference between a girl and a boy.

All embryos have a small bud or swelling (the genital nub).

If you’re having a boy, testosterone starts being produced at seven weeks. This prompts the bud to grow and develop into a penis and scrotum. In a girl, the genital nub will become the clitoris and labia.

Nub theory is based on the idea that it’s possible to tell which type of genital nub your baby has by looking closely at the images from your dating or NT scans. But nub development is a gradual process, and it’s a real challenge to see which way the nub is developing around the time of these scans. This is why it’s so difficult to work out a baby’s sex in the first trimester.

These ultrasound images were taken at 13 weeks of pregnancy, and show the genital nubs of two different babies.


The first baby has a genital nub at an angle that’s greater than 30 degrees, so he’s probably a boy. The nub is also beginning to look like a penis and scrotum.

The second baby has a genital nub that’s more horizontal to the baby’s body, parallel with the spine, so she’s probably a girl.

In terms of nub theory, these scans provide an unusually good view of the babies’ emerging genitals on an early scan. Usually, first-trimester scan images are much less clear.

This baby is between 11 and 13 weeks. This nub is in a horizontal position, parallel with the spine, so could be female. But the nub’s shape is more like a boy’s.

It’s best to wait until your anomaly scan, which you’ll be offered between 18 and 20 weeks. Getting your baby’s sex correct at this stage is far more likely.

If you can’t wait until your 20-week scan, you can pay to have a non-invasive prenatal test (NIPT) from when you’re 10 weeks pregnant.

NIPT is a blood test that screens your baby for genetic abnormalities, such as Down syndrome. It can also tell you the sex of your baby. NIPTs aren’t covered by Medicare or private health insurance, though. Read our article to find out more about non-invasive prenatal tests (NIPTs).

More about the sex of your baby

• Watch how your baby’s genitals develop
• Discover four ways to find out your baby’s sex for sure
• Try our fun Chinese gender predictor

Reviewed for BabyCenter Australia by Peter Muller, obstetrician and maternal-fetal medicine specialist.