Nub theory explained: 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.
  • We earn a commission for products purchased through some links in this article.
  • What exactly is the nub theory - and how can it help to predict the sex of your baby before the five month scan?

    We’ve all heard of the old wives tales like carrying your baby bump low means it’s a boy, or if you’re craving sweet things, you’re expecting a girl.

    If you’re looking for the nub theory explained, or want some more ideas on how to tell the sex of your little one, you’ve come to the right place.

    The nub theory claims to be a lot more accurate than some of those old wives tales – and claims you can even tell the sex of your baby just by looking at your 12 week ultrasound picture. Usually the earliest point you can tell the sex of a baby from a scan is 20 weeks, so the quirky theory is popular with those who can’t wait to find out if they’re having a little girl or boy.

    What is nub theory?

    Between 11-13 weeks, all babies are said to have a ‘nub’ between their legs called the genital tubercle, and according to this theory the angle of the nub will indicate whether it’s a boy or girl.

    It’s said that if the nub is over 30 degrees up from the spine, it’s a boy – but if it’s under that then it’s a girl.

    Confused? See our pictures below as an example.

    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.

    Not convinced? Netmums have reported the following statistics inline with the nub theory:

    • At 11 weeks, the accuracy rate is 48 per cent
    • At 12 weeks, the accuracy rate is 91 per cent
    • At 13 weeks, the accuracy rate is 94 per cent


    Going for your scan soon? You’ll have to hope for co-operation from your baby as some professionals say the accuracy actually all depends on the angle of the baby.

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

    What do the experts say about nub theory?

    “A 2016 study looked at the accuracy of [nib theory]. The researchers found that out of 672 cases, sex determination was possible 90 percent of the time, and the prediction was correct 87 percent of the time. While those percentages may seem pretty high, it’s not as easy as it sounds.” Patricia Santiago-Munoz, M.D. says. 

    “There are a number of variables that can affect whether we can determine gender as early as 12 weeks. 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.

    “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 I mentioned 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.

    “There’s also a chance that our prediction simply will be wrong. We tend to overpredict 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. 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.”