However, the nub theory claims to be a lot more accurate - and says 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 a little 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
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
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:
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.
If you had success using the nub theory then we'd love to know! Leave us a comment below or send us a message on ourFacebook page.