There are a number of different techniques and theories that claim to be able to tell you the sex of your baby early in the pregnancy. One that you might have heard of is the ‘nub theory’. This claims to be a way of telling whether your baby is a boy or girl at about 12 weeks from an early ultrasound scan.
What is my baby’s nub?
The ‘nub’ in question is a little nub of tissue called the ‘genital tubercle’. This is what will go on to develop into a penis in a boy, or a cl*tor*s in a girl. During the first few weeks, however, this nub looks the same whether your baby is a boy or a girl.
What is nub theory?
Until about 13 weeks of pregnancy, the genitals of a boy look much the same as those of a girl.
Nub theory involves trying to identify the sex of the baby by looking at the way this nub starts to point, as captured on an ultrasound scan. It’s believed that the way the nub points can tell you whether the baby is a boy or a girl; in a boy the nub points up towards their head, and in a girl it points down towards their feet.
Using this theory, it may be possible to tell a baby’s sex from about 11 or 12 weeks, before their genitals have clearly developed.
What a boy’s nub looks like
Nub theory relies on looking at how the nub is positioned relative to the developing spine. This means you will need to have a clear profile from the ultrasound showing both the spine and the nub. If the nub points upwards and away from the spine - specifically, if the nub is at an angle of 30 degrees or more form the spine – this may indicate that you are having a boy.
What a girl’s nub looks like
If the nub is pointed downwards or is parallel to the spine, this may indicate that you are having a girl. Specifically in this case you should be looking for an angle of less than 30 degrees. There is some room for error though, especially if the nub is very close to that magical 30 degree angle – the more it points upwards (away from the spine) or downwards (along the line of the spine) the clearer the difference should be.
Does nub theory work?
Nub theory is certainly not a guaranteed way to tell the sex of your baby but there does appear to be something in it. One small study looked at predicting the baby’s sex by identifying the angle of the genital tubercle (or nub) and measuring whether it was greater or less than 30 degrees to the horizontal line.
The accuracy was found to be 70.3% at 11 weeks rising to to 98.7% at 12 weeks and 100% by 13 weeks. Interestingly, at the 11-week mark, more than half (56%) of male foetuses were wrongly identified as female, while only 5% of females were wrongly identified as male.
Can I find out the sex of my baby at my first scan?
Your first NHS scan is generally referred to as the ’12-week scan’, although it can happen anywhere from 11-14 weeks. This has a number of purposes including:
- Dating the pregnancy
- Checking the size and development of your baby
- Checking if there is more than one baby (twins, triplets or more)
- Checking the position of the baby and placenta
- Early detection of some physical conditions
It is NOT designed to tell the sex of your baby though and this is not offered as part of the national screening programme.
It may be possible during the 20-week screening scan, but this depends on the policy of your hospital – some have a policy of not predicting a baby’s sex. You should also be aware that even at this stage it is not possible for the sonographer to be 100% certain about your baby's sex. If your baby is lying in an awkward position, for example, it may be difficult or impossible to tell.
Should I try working it out for myself?
If you have a very clear scan showing the line of your baby’s spine and the position of the nub, you can try to work out whether the nub is more than 30 degrees from the spine – and therefore whether you might be expecting a boy or a girl.
There are no guarantees though. In the study mentioned above the predictions were being done by trained professionals with clear scans, which can be difficult to get.
Some places offer private scans and/or interpretation of existing scans. They claim high rates of accuracy at an early stage but no one claims 100% accuracy (and if they do you should probably be wary!)
