Finding Out the Sex of Your Baby by Ultrasound

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When can you find out the sex of your baby by ultrasound?

baby ultrasound
Charles Gullung/Cultura RM/Getty Images

If you have chosen to find out the sex of your baby, you are most likely going to do so via ultrasound. Typically an ultrasound is done for this purpose around the midpoint in pregnancy. It is technically called the fetal anatomy survey and it is done to look for fetal anomalies and not simply the sex of the baby. Most women will have one between 18-22 weeks of pregnancy.

Many families rely on ultrasound to tell the gender or sex of their new baby. How accurate that report is will depend on many factors including the age of the baby, the equipment used, the technician, and the baby cooperating.

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Absence of Penis = Girl Theory

23 Week Ultrasound - It's a girl!
23 Week Baby Girl. Photo © J. Ashworth

One of my pet peeves is that people take ultrasound for granted when it comes to gender or sex determination. Not only can ultrasound be wrong, but there is also what I call the "Absence of Penis = Girl" theory. This theory states that if the ultrasound technician fails to find a penis the baby is automatically a girl. You will see that clearly a girl is more than the absence of a penis.

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The Hamburger Sign (Girl)

It's a girl! Twin B - 18 Week Ultrasound
18 Week Baby Girl. Photo © M. Horn

When doing an ultrasound to determine the sex of your baby, an ultrasound technician will actually look for girl genitalia - labia and clitoris. When these are seen it is often referred to as the "Hamburger Sign" because the clitoris between the labial lips looks hamburger-like. Where the labia would be the buns of the hamburger and the clitoris would be the meat.

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The Turtle Sign (Boy)

It's a boy! - 19 Week Ultrasound
19 Week Baby Boy. Photo © C. Baker

When the ultrasound technician is looking for a boy, they are looking for something called the turtle sign. This is where you can see the tip of the penis peeking out from behind the testicles. This may be harder to tell some babies than others, which is why there are multiple signs to look for during an ultrasound.

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The Erect Penis

20 Week Ultrasound - Boy
20 Week Baby Boy. Photo © C. Filipek

Even in the stages of fetal development baby boy's can and will have erections. If you happen to be looking with an ultrasound during that time frame you will see a very clearly defined penis. This may bother some parents, don't worry, it is normal for boys to have erections. You will notice these after birth as well during diaper changes.

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Age Makes a Difference

2nd trimester ultrasound
Photo © K. Harrell

When doing ultrasound we're working really hard to be able to tell a parent the sex of their baby earlier and earlier. The problem is that the earlier you are told the more likely the ultrasound technician is to be wrong. Ultrasound before 18 weeks runs this risk because the genitals of boys and girls are easily mistaken, namely because the clitoris is roughly the size of the penis at this stage.

It's hard to tell at 15 weeks, but the technician's best guess is a boy.

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How 3D Ultrasound Works for Girls and Boys

35 Week Baby Girl
35 Week Baby Girl. Photo © REW

The greater availability of 3D ultrasound has changed how ultrasounds for your baby's sex is looked at during an exam. These are often more accurate because you can actually see parts as opposed to guessing from shadows. The images are often very clear and very distinct.

No matter what type of ultrasound that you use, there will always be errors. The errors may be human or technical, even when rare, it can still leave you feeling pretty shaken when the daughter you were expecting turns out to be a baby boy.

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