And it's the method used by sonographers (the qualified professionals who use ultrasound imaging to conduct and interpret scans) at your 20-week NHS pregnancy scan and also, if you wish to pay for extra scans, at private scan clinics.
Here's how to spot the 3 lines, know what they mean, and tell the difference between a boy and a girl ultrasound image
What are the 3 lines on an ultrasound?
The 3 lines are the image you can see on the scan of a baby girl's genitals: the cl*tor*s, surrounded by the 2 lips of the labia. So if there are 3 lines on your pregnancy scan, you're pregnant with a little girl.
"Sonographers look for the distinguishing signs of the different genitalia," says Professor Lees. "With a female baby, this can appear to look like 3 white lines.
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"With a male baby, it is often possible to observe the penis, testicl*s and scrotum instead."
Is this the same as the 'hamburger' sign?
Yes, the 3 lines can also be described as looking like the outline of a hamburger (bun, burger patty, bun – in a stack).
And is this the 'potty shot' image?
Yes, the scan image that shows the presence – or not – of the 3 lines can only be captured from a view up between your baby's legs, why is why it's often called the potty shot.
"It's the view you'd get if you positioned a camera in the base of a potty and your baby was sitting on top of the potty," says Jan Steward, Director and Co-Founder of the private ultrasound provider Ultrasound Direct.
"We give all parents-to-be who come for scans some example 'potty shot' scan images, and explain that looking for the 3 lines is one of the ways that we are able to determine if you are having a girl."
What do the 3 lines look like on an ultrasound that are the sign of a girl baby?
On the pregnancy ultrasound scan picture above – that's been shared with us by a member of our MadeForMums Top Testers Club community – you can see the 3 lines that indicate a girl baby.
Scan pictures are often a little blurry, we know: what you're looking for is the baby's genital area which is just underneath the white arrow.
If you need some help getting your head around this view of the baby, look at the black area at the centre of the picture; now, find the baby's 2 feet above and below this black area, then track left from the feet up the legs and you'll arrive at the genitals, as seen from below (the 'potty shot' area).
Here are two more scan pictures, shared with us by different members of our community, each showing the 3 lines that indicate a girl baby. With the picture above left, the baby is positioned with their feet in the centre and their legs, leading up to the genital area, on the right; with the picture above right, you can see the 3 lines indicated by the white arrows.
It you're struggling to see the 3 lines – we know it's not completely obvious – we've also got a picture, below, that Ultrasound Direct share with their customers who pay for a private scan.
"We've circled the 3 lines in these scan images," says Jan Steward, "so you can clearly see them. We give this to parents-to-be when they come for their scan so that they can see what we are looking for when we determine sex."
What does a boy ultrasound scan look like? What are the signs of a boy?
This is a picture of a pregnancy ultrasound image, shared by member of our MadeForMums Top Testers Club community, of a boy baby. It's clearly different from the girl scan images above: no 3 lines and you can see the outline of male genitalia sticking out between the legs (right in the centre of the picture) instead.
And here are two more 'boy' scan pictures, shared with us by different members of our community, each showing the clear outline of a penis and scrotum between the legs. With the picture above left, you can see the legs on the right of the image with the feet pointing up towards the middle; with the picture above right, the baby's legs are sticking out straight across the middle of the image, with the feet (not visible in their entirety) on the left and the 'potty shot' in the centre of the image.
And here, below, is the example picture that private scan provider Ultrasound Direct shows to their customers, so that they can see what their sonographers are looking for in the scan image as a marker of a boy baby.
How accurate is the 3 lines method at telling the difference between a boy and a girl?
It's usually pretty spot-on but it's not 100% foolproof. "It may not be possible," says Professor Lees, "to be fully certain about the baby's sex because the baby may be in a position that will make it difficult for the sonographer to be sure."
And Jan from Ultrasound Direct agrees. "We give a professional opinion," she says, "and it's usually an accurate one but there are a few cases when there is a surprise. We're afraid there is no such thing as 100 per cent."
Can you use the 3 lines method on a 12-week pregnancy scan? Or any scan before 20 weeks?
It's not possible to see the 3 lines on a 12-week scan – and it's very tricky to see them before about 14 or 15 weeks. Up until then, what you can see on a potty shot looks pretty much the same, whether your baby's a boy or a girl.
That's because, although your baby's sex is determined at the moment of conception, their external genitals don't begin to develop until about 9 weeks¹ and, at this early stage, they're just a genital tube – the (very similar) beginnings of what will gradually develop, by about 11 weeks, into either a penis and scrotum or a cl*tor*s and labia.² It can take until 14 or 15 weeks to be able to clearly see this difference on an ultrasound scan.1,2
So, if you want to look for the 3 lines, you'll need to wait for your 20-week NHS pregnancy scan or book a private scan for between 16 and 23 weeks or so.
"We don't offer to tell you if your baby's a boy or a girl before 16 weeks," says Jan from Ultrasound Direct, "because the 3 lines are only visible from 16 weeks.
Can you see the 3 lines on a 3D or 4D scan?
Actually, probably not – both because this type of scan makes it harder to tell, and because this type of scan is usually done later on in pregnancy when it's harder to tell anyway.
"Despite what you might think," says Jan, "it's is a lot harder to spot the sex on a 3D or 4D scan. Also, the optimum time for these scans are between 28 weeks and 32 weeks, and by this stage your baby has become chubbier and, if they are a girl, their labia will be more swollen, making the 3 lines difficult to visualise.
"What's more, at this stage of pregnancy, if a baby boy has his legs together, the testicl*s can get squeezed and look similar on the scan to a girl's labia. It is definitely easier to tell earlier on in a pregnancy!"
Will I be able to see the 3 lines on the picture I get after my 20-week scan picture?
It depends. Sometimes, you may get a printout of a quite magnificently clear potty shot; sometimes, it's all a bit blurry and you can't make out what the sonographer was seeing. And sometimes, of course, your scan picture shows your baby in a different position altogether.
About our expertProfessor Christoph Lees
Professor Christoph Leesis Head of Fetal Medicine at Imperial College Healthcare NHS Trust. He qualified from Guy's Hospital, London, in 1990, with a sub-specialty accreditation in fetal-maternal medicine under Prof Kypros Nicolaides at the Harris Birthright Centre for Fetal Medicine, King's College Hospital, London. He established the fetal medicine unit at Addenbrooke's Hospital, Cambridge and was Head of Fetal Medicine at Queen Charlotte's and Chelsea Hospital. He is an international authority on fetal assessment and the use of Doppler ultrasound to assess the health of the baby. He leads research into high-intensity focused ultrasound and is a committee member of theInternational Society of Ultrasound in Obstetrics and Gynaecology.
About our expertJan Steward
Jan Stewardis a qualified ultrasound professional. She is the Director and Co-founder ofUltrasound Direct, a national network of healthcare professionals that employs over 250 sonographers to offer a professional and personal private ultrasound service. Ultrasound Direct Ltd is registered with the Care Quality Commission CQC and is the largest employer of sonographers outside the NHS.
References:
1. 'Macroscopic Whole-Mounts of the Developing Human Fetal Urogenital-Genital Tract: Indifferent Stage to Male and Female Differentiation.' Shen et al. Differentiation. 2018 Sep-Oct; 103: 5–13. Published online 2018 Aug 30. doi: 10.1016/j.diff.2018.08.003
2. 'Embryology. Sexual Development.' Aatscha, PA et al. Stat Pearls. Last update August 2023.
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