If you want to know your baby's sex before it's born, the 3 lines method is a tried and tested way sonographers use to tell if you're having a boy or girl.
We caught up with Professor Christoph Lees, Consultant Obstetrician and Spokesperson for the Royal College of Obstetricians and Gynaecologists on this one, and he told us:
“The commonest and most reliable way to determine a baby’s sex during pregnancy is during an ultrasound scan, usually performed from 18-21 weeks on the NHS.
"During this ultrasound scan, sonographers are looking for the distinguishing signs of the different genitalia.
"With a female baby this can appear to look like three white lines. With a male baby, it is often possible to observe the penis, testicles and scrotum at the second trimester routine scan."
What are the 3 lines on an ultrasound?
“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.
3 lines - girl example
"The picture below is of a 20-week scan of a girl and clearly shows the 3 lines.
"When the sonographer points out why she thinks it is a girl she will use these 3 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.”
3 lines - boy example
This 20-week scan, posted by forum user SW2, shows a boy. It looks very different from the 3 lines girl scan above, with the genitalia clearly sticking out.
How accurate is the 3 lines ultrasound method?
It's not always right, says Professor Lees. "Even during the second trimester routine scan it may not be possible to be fully certain about the baby’s sex because s/he may be in a position that will make it difficult for the sonographer to be sure."
Indeed, as the Professor says, 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, Professional Officer at Ultrasound, “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.”
Though Jan agrees with Nigel, saying: “We give a professional opinion but there are a few cases when there is a surprise. There is no such thing as 100 percent."
At what age/stage can you see the 3 lines on an ultrasound?
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.”
On our forum, 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 the 3 lines 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."
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," says snappyloz on our forum. "I said it to them before they said it to me, but there was no sign of it on the picture."