It's a long old wait until that 20-week pregnancy scan, when you'll get the chance to find out whether you're having a girl or a boy. If you're keen to know your baby's sex sooner, there are plenty of 'gender-prediction' tests and theories to try – of which the most well known is probably the nub theory.


According to the nub theory, you can predict your baby's sex as early as 10 to 13 weeks, just by looking at a side-shot ultrasound picture, like the one you'll get at your dating scan, spotting your baby's developing genitalia and then calculating what's known as the 'angle of the dangle'.

What is the nub theory?

"Early on in pregnancy, before a baby's genitals are formed," says expert family GP Dr Philippa Kaye, "a bump or nub develops between their legs. This is called the genital tubercle or genital tube.
"This nub develops into a penis, if your baby is a boy, or into a clitoris, if your baby is a girl. This process starts between about 8 or 9 weeks but it's gradual, and the external appearance of the developing penis or clitoris stays roughly the same till about 14 weeks."
The basis of the nub theory is that there is enough of a difference in development at 10 to 13 weeks to be able to spot on an ultrasound whether the nub is turning into a penis or clitoris.

How can you tell? It's all about the angle of the nub in relation to the lower part of the baby's spine, so the theory goes:

  • The sign of girl: the spine and nub are in line or at an angle of less than 30°
  • The sign of a boy: the angle of the nub to the spine is more than 30°

When can you use the nub theory?

Nub Theory fans would say you can use the theory on picture of ultrasound scan that's been performed between 9 weeks and 13 weeks of pregnancy – and that the theory works most effectively from about 12 weeks.

More like this

What does a girl nub look like? Here's an ultrasound scan example

example of nub theory on an ultrasound scan predicting a girl

We asked our Top Testers Club community if any of them had used the nub theory on their 12-week dating scan pictures. And here (above) is the scan pic Kirsty M sent us.

We've labelled the spine and genital tubercle and drawn on the yellow lines to show an angle of less than 30°, indicating, according to the nub theory, that this baby is a girl.

"And, in our case, the theory was spot on," says Kirsty. "I did go on to have a baby girl!"

What does a boy nub look like: ultrasound example

example of nub theory predicting a boy from an ultrasound scan

Here (above) is a 12-week scan picture from Emily S, who's also a member of our Top Testers Club community.

Again, we've labelled the spine and genital tubercle and drawn on the yellow lines to show an angle of more than 30°, indicating, according to the nub theory, that this baby is a boy.

And did that turn out to be correct? "Yes," says Emily. "A nub theorist analysed this for us and said they were 90% certain it would be a boy – and they were absolutely correct."

How accurate is the nub theory?

Ah well, as fun and interesting as all the line drawing and angle calculation is, it is only a theory – and there's really no solid scientific research behind it.

That's not to say there's nothing in it: there have been several studies conducted using it, with varying limitations, but they've all shown it to have varying degrees of accuracy.

Back in 1999, a small study of 172 pregnancies¹ found the nub theory accurately pinpointed the sex of 70% of the babies at 11 weeks, 98% at 12 weeks and 100% at 13 weeks. The same lead scientist repeated the study on a larger scale² (656 pregnancies) in 2006, with slightly less accurate results (85% accuracy at 12 weeks and 97% at 13 weeks).

A more recent – and much larger – study in 2012 (1222 pregnancies)³ reported an even lower accuracy rate of 30% at 11 weeks, although this did rise to 96% at over 12 weeks.

This all sounds reasonably promising, especially if it's done at 12 or 13 weeks – until you realise that, in all these studies, the measurements were taken during an ultrasound examination and analysed by ultrasound professionals.

"There is no evidence that the nub theory works reliably when done by parents-to-be looking at their ultrasound pic printout," says Dr Philippa.

Gill Harrison, professional officer for ultrasound at the Society and College of Radiographers, agrees. "It's extremely difficult even for professional sonographers to be accurate about assessing a baby's genitalia at the early scans," she says. "And that's assuming the baby is in the right position when you perform the ultrasound.

"We would not use nub theory at 12 weeks to predict a baby's sex.

"It makes sense to wait for your 20-week pregnancy scan and get a more confident indication of whether you're having a boy or a girl."

How else can I tell my baby's sex?

As Gill says, you'll get the most accurate news about your baby's sex from your 20-week pregnancy scan. But while you're waiting for that, you could try out some of the other gender-prediction theories:

You could also look up our Chinese gender prediction chart and check out all these other ways to 'tell' if you've having a boy or a girl.

About our expert Dr Philippa Kaye

Dr Philippa Kaye works as a GP in both NHS and private practice. She attended Downing College, Cambridge, then took medical studies at Guy’s, King’s and St Thomas’s medical schools in London, training in paediatrics, gynaecology, care of the elderly, acute medicine, psychiatry and general practice. Dr Philippa has also written a number of books, including ones on child health, diabetes in childhood and adolescence. She is a mum of 3.

About our ultrasound expert Gill Harrison

Gill Harrison is a highly experienced sonographer and radiographer, currently working as the Professional Officer (Ultrasound) at the Society and College of Radiographers. She was previously an Associate Professor at City, University of London and responsible for leading the medical ultrasound programme. She is a lead accreditor and external examiner, assessing the competency of ultrasound students and qualified sonographers in UK universities and hospitals. She is Chair of the Health Education England's Sonographer Training Group and was previously Chair of the European Federation of Radiographer Societies' Working Group for Ultrasound Surveys (2019 to 2022) and Chair of the Society and College of Radiographers' Ultrasound Advisory Group.


1. First-trimester determination of fetal gender by ultrasound. Efrat Z et al. Ultrasound in Obstetrics and Gynecology. 1999 May;13(5):305-7. doi: 10.1046/j.1469-0705.1999.13050305.x
2. Fetal gender assignment by first-trimester ultrasound. Efrat Z et al. Ultrasound in Obstetrics and Gynecology. 21 February 2006.
3. Ultrasound Evaluation of Fetal Gender at 12 to 14 weeks. Lubusky et al. Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub. 2012, 156(4):324-329. doi: 10.5507/bp.2012.022

Pics: With thanks to Emily and Kirsty in the Top Testers Club. Graphics: Jordan Edmonds-Moore


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Helen Brown
Helen BrownHead of Content Delivery

Helen is author of the classic advice book Parenting for Dummies and a mum of 3. Before joining MadeForMums, she was Head of Community at Mumsnet and also the Consumer Editor of Mother & Baby.