Nub theory: How to predict your baby’s sex at 12 weeks

Nub theory - have you heard of it? It can help you find out if your baby is a boy or girl from just 12 weeks.
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  • The Nub Theory is a popular way for parents-to-be to predict their baby’s gender at just 12 weeks.

    Unlike other methods that use astrology to predict a baby’s gender or old wives tales about pregnancy to try and guess if you are carrying a boy or a girl, the Nub Theory uses the 12-week ultrasound scan picture to predict the sex of your baby more accurately.

    Usually the earliest point you can tell the sex of a baby from a scan is 20 weeks, but like the Ramzi Theory, this quirky theory says you can tell the sex of your baby at your very first ultrasound. So it’s popular with those who can’t wait to find out if they’re having a little girl or boy.

    What is Nub Theory?

    The Nub Theory is based on the hypothesis that you can predict the sex of your baby based on the angle of their genital tubercle. All babies have a genital tubercle or ‘nub’ between their legs that develops between 11-13 weeks.

    According to this theory, the angle of the nub will indicate whether a baby is a boy or girl. It’s said that if the nub is over 30 degrees up from the spine, it’s a boy. However, if it’s under that, then it’s a girl.

    A 2016 study looked at the accuracy of the Nub or ‘Nib’ theory and the results were surprisingly accurate. The researchers found that out of 672 cases, sex determination was possible 90 percent of the time and the baby’s gender was correctly predicted 87 percent of the time.

    See our pictures below as an example of how to use the Nub Theory to predict a baby’s gender.

    Nub theory indicating a girl

    Nub theory - girl picture
    The nub isn’t pointing up more than 30 degrees from the baby’s spine – in fact it’s almost horizontal – so believers would say this showed the scan to be a little girl.

    Nub theory indicating a boy

    Nub theory - boy picture
    The nub is at a much steeper angle upwards from the spine, indicating that this could be a baby boy.

    What do the experts say about nub theory?

    “While the percentages [of those who accurately guess their baby’s gender using the Nub Theory] may seem pretty high, it’s not as easy as it sounds.” says Patricia Santiago-Munoz, M.D.  “There are a number of variables that can affect whether we can determine gender as early as 12 weeks.”

    Patricia explains that you’ll have to hope for co-operation from your baby during the 12-week scan, as getting a good enough photo to use for the Nub Theory all depends on the angle of the baby:

    “First, your baby needs to be in a position for us to get a good photo. That includes not having their legs closed! Keep in mind, the sonographer may not have extra time between appointments to wait for the baby to move into a better position to make a prediction.”

    If you can get a picture of them laying down on their side, that should provide you with a good view of the nub.

    Patricia also explains that the quality of the ultrascan photo may also be affected by other factors too.

    “A mother’s weight also affects our ability to predict gender. The more body tissue the ultrasound waves must travel through, the fuzzier the images may be. The 2016 study found that a body mass index below 23.8 was the best cutoff value for gender prediction at 11 to 13 weeks. The odds of an accurate prediction fall for women above that number.

    Woman holding an ultrascound picture after using the nub theory to predict her baby's gender.

    (Credit: Getty)

    Can the Nub Theory be wrong?

    Yes, the Nub Theory can definitely be wrong. Despite the high percentage of correct gender predictions from the study in 2016.

    “There’s a chance that our prediction simply will be wrong,” says Patricia. “We tend to over-predict boys more often than girls. This can happen, for example, if the baby is developing slowly and the tubercle hasn’t begun to point up or the umbilical cord is mistaken for a penis.

    “While gender prediction is much more accurate during the 20-week ultrasound, there’s still a chance it can be wrong. I recently had a patient who was expecting a boy. She and her husband had a name picked out and had painted the nursery for a boy when they found out shortly before giving birth that they actually were having a daughter. They were shocked but took the news in stride.

    “I want to stress that the main goal of blood tests and ultrasounds during the first trimester or early second trimester is not to determine gender.”

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    “Ultrasounds are used to screen for certain conditions and check whether there is something noticeably wrong with your baby.

    “While we understand you may be eager to learn the gender, try not to be upset if your sonographer can’t predict it.”

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