How does a sonographer use the 3 lines at your 20-week scan to tell if you're having a boy or a girl?

We know there are some pretty alternative ways to find out your baby’s sex, from nub theory to skull theory to the super-fun (highly non-scientific) pendulum test (even Jools Oliver tried this one) - and then there's the 3 line method - which identifies the labia to determine a girl.

Whereas options like nub theory or the Chinese baby sex prediction chart aren’t necessarily ones that the medics are on board with, using the 3 lines method is a tried and tested way sonographers use to work out your baby's gender.

Ooooh, now we're interested.... 😀

What are the 3 lines?

“It basically identifies the labia,” confirms Nigel. “The 3 lines just describes the upper part and the lower part and is a good way of determining female.”   

“Looking for the 3 lines is one of the ways that we are able to determine if you are having a girl,” agrees sonographer Jan Steward, Director of Ultrasound Direct.

“We give all our parents that come for scans some example ‘potty shot’ photographs of what we are looking for when we determine sex.

"The picture is of a 20-week scan and the one of the girl clearly shows the three lines.

"When the sonographer points out why she thinks it is a girl she will use these three lines as an illustration.”

And hopefully this will be clearly seen on the screen when you're having the scan. 

Lots of the mums on our forum have experienced sonographers using this method when they have scanned them. As fall3n-ang3l told us: “When I was having twins I was having a boy and a girl.

"The girl has 3 lines and when the sonographer showed us you really could tell the difference.”

At what age/stage can you see them?

Within the NHS this method is generally used as part of determining sex at the scan everyone is offered at around 20 weeks. At private clinics it is usually used as a method from 16 weeks.  

“The 3 lines are visible from 16 weeks because the baby hasn't got any muscle or fat deposits,” explains Jan.

“As the pregnancy progresses and the baby becomes chubbier, the labia begins to swell and the same 3 lines we see at 16 weeks now swell up from the mid-trimester onwards.

"It can then become more difficult to be sure because if a little boy has his legs together the testicles can get squeezed and look similar. It is definitely easier to tell earlier on in a pregnancy than later.”

HappytoHook shares her experience of the 3 lines method at around 20 weeks.

“She sexed the baby for us, describing what she could see and checked 3 times! Three little lines of the girly bits - a hamburger she called it! She wriggled baby a bit to make sure no boy bits were hiding!”

Can you see them on a 3D/4D scan?

As a general rule sonographer’s will not use 3D/4D scans for sexing your baby. This is mainly due to these scans being done much later in pregnancy when it’s harder to tell.  

As Jan explains: “Despite popular belief it is a lot harder to tell the sex on a 3D or 4D scan. 4D is dependent on foetal position and amniotic volume. Also, at the optimum time for 4D at 28 weeks, the genitalia can be difficult to visualise.”

Will I always be able to see the 3 lines?

“It is not always possible to see these 3 lines,” warns Nigel. “The position of your baby is obviously a factor. If they are clearly visible, however, it is a good way to determine sex.”

And while the sonographer might be using it as a method to check, those 3 lines might not be immediately obvious to you. Certainly not perhaps as clear as boy bits!

Lynz_81 shares her advice: “Try to have a good look between the legs. Boys are usually pretty easy to see, my 2 looked like little snails.

"Girls have 3 lines between their legs. However, it is hard to tell when you're not sure what you're looking at, and baby might have its legs crossed, or be moving around too much for you to get a good look.”

We think that ultimately the 3 lines would really need to be pointed out to you by the sonographer, and even then it’s not always obvious, as LH86 discovered.

“It’s really not easy to tell! I saw a line and was convinced it was a girl but it could have been a girl's private parts or just the line where her legs meet if they were closed.

"And actually the best the sonographer could tell us was: 'I’d say it’s more likely to be a girl but the picture isn’t very clear I’m afraid.'"

Will I see the 3 lines on the scan picture after?

In some cases you may get quite a clear illustration of the 3 lines, not so much in others. 

For some of our mums, even when it was blatantly obvious in the scan room that they were having a boy, with a willy clearly showing, it didn’t come out on the picture at all.

“We wanted to know the sex of our baby and there was no mistaking our strapping young man in the scan room. I said it to them before they said it to me, but there was no sign of it on the picture,” says snappyloz

Is this method always accurate?

There are no guarantees when it comes to sexing your baby. In ‘Can your baby’s sex be wrong?’ we reveal at least one occasion where the new arrival isn’t exactly what mum was expecting.

And you might remember this pic - which went viral - showing Koto Nakamura, who was told she was having a girl and actually gave birth to a boy....

As with most things in pregnancy, it’s never entirely predictable. But, having spoken to sonographers, we think identifying the 3 lines is a pretty safe bet on you having a girl.  

That said while the 3 lines is definitely a tried and tested method, sonographers themselves will always be a little bit cautious.  

“Even if the 3 lines are identified it is still only an opinion from the sonographer,” says Nigel Thomson “albeit a reasonably accurate one.”

K Lou, however, found that her sonographer at a private scan was happy to be pretty definitive.  

“I had a private scan at 16 weeks and I was told I was having a girl. She said she would eat her hat if it was a boy and showed me the 3 lines.”

“We give a professional opinion but there are a few cases when there is a surprise. There is no such thing as 100 percent," says Jan.

But we would say, compared to some of the other methods out there - this seems a pretty safe way to go 😀

And, in the end, it’s up to you how far you go in terms of what you buy before your little one comes along.  

lynz_81 has been out buying lots of pink: “The guy was really definite! Must admit I had a good look too and it definitely looked like a girl to me!

"He said it was a really good picture and u could see the 3 lines very clearly.  

"I’ve bitten the bullet and gone all out on the pink stuff. If it turns out to be wrong then - oh well - our little boy will just have to like pink dresses!”