Nub Theory: What is it and how it predicts your baby’s sex at 12 weeks

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  • The Nub Theory can help you discover your baby’s gender at the 12 week scan, all you need is your first ultrasound picture.

    Pregnancy is such an exciting – but slow – time! During that first trimester you’re excited to know as much about your baby as possible; like the gender. Usually you have to wait until your 18-22 week anatomy scan to be in with a chance of finding out, but 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 guess if you’re 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.

    Similar to the Ramzi Theory, the nub 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.

    Nub Theory: what is it?

    The Nub Theory is all about something called the genital tubercle, or a ‘nub’. All babies have this between their legs and it develops between 11-13 weeks, turning into a penis in males and a clitoris in females.

    The idea is that, if you can get a clear view and baby isn’t doing fetal gymnastics, you can predict the sex of your baby based on the angle of the nub. According to this theory, the ‘angle of the dangle’ 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.

    To be able to apply the Nub Theory your baby needs to be in a clear profile position, so the length of their spine is visible.

    Nub Theory 12 weeks: boy

    Image of a scan highlighting the nub of a boy

    Illustration highlighting nub theory for a little boy

    Credit: Canva

    The nub is at a much steeper angle upwards from the spine, indicating that this could be a baby boy. Now, we’re not sure what 30 degrees looks like on an ultrasound. But if you were to draw a straight line on the ultrasound along the bottom half of your baby’s spine (near their bottom) you can see whether the nub is pointing clearly up and away from that line or not.

    Nub Theory 12 weeks: girl

    image of a scan with an arrow pointing to a horizontal nub indicating a girl

    Illustration of the nub theory on a baby girl

    Credit: Canva

    If you compare the angle of your baby’s bottom/spine to the angle of the nub, and it sits horizontally in line with the spine or even points down towards it, believers would say this shows a little girl.

    Nub Theory accuracy: what the experts say

    The Nub Theory, does what it says on the tin. It’s a theory. While it may sound a little wishy-washy, there are studies to back it up. 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.

    “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.”

    Pregnant woman with yellow dress holding ultrasound scan for her unborn baby.

    Credit: Getty Images

    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!” 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 ultrasound 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.”

    Want to give it a go yourself? Here’s an easy video on how to do it, all you need is your ultrasound photo;

    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.”

    “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.”

    In conclusion, the Nub Theory is a fun way to guess your  baby’s gender after your 12 week scan. Though, probably best to wait until a medical professional has confirmed your baby’s sex. Or until you’ve met your baby in person, before committing to any kind of colour scheme or wardrobe.



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