Finding Out the Sex of Your Baby by Ultrasound

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

baby ultrasound
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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 eighteen and twenty-two 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.

In many ways, the more traditional method of doing a mid-pregnancy ultrasound is something that is becoming more of a confirmation than the actual moment you find out. This is because more and more people are finding out the sex of their baby earlier in pregnancy, from a variety of ways. This can mean that there is a lack of surprise or feeling of being let down because of potential disappointment at the sex of the baby.

Absence of Penis = Girl Theory

ultrasound in pregnancy
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A pet peeve of many ultrasound technicians 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 is called 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, clearly, there is more to a girl ultrasound picture than the absence of a penis. In fact, particularly early on in pregnancy, a clitoris and a penis are roughly the same size and shape.

The Hamburger Sign (Girl)

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 situated between the labial lips looks like a hamburger between two buns, or three lines. Where the labia would be the buns of the hamburger and the clitoris would be the meat.

The Turtle Sign (Boy)

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. The gestational age and position of the baby play a part into what they can see during the ultrasound as well. This is why timing these ultrasounds is so important. 

The Erect Penis

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 obviously makes identifying a baby boy much easier than when the penis is not erect. Seeing an erection 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.

Age Makes a Difference

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 eighteen 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.

In the later part of the first trimester, the ultrasound is used to look at the direction of the genital tubercle rather than the actual penis or clitoris. Whereas in later ultrasounds, the actual external genitalia is looked at via ultrasound.


How 3D Ultrasound Works for Girls and Boys

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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Manzanares S, Benítez A, Naveiro-Fuentes M, López-Criado MS, Sánchez-Gila M. Accuracy of fetal sex determination on ultrasound examination in the first trimester of pregnancy. J Clin Ultrasound. 2016 Jun;44(5):272-7. doi: 10.1002/jcu.22320. Epub 2015 Dec 11.

Odeh M, Granin V, Kais M, Ophir E, Bornstein J. Sonographic fetal sex determination. Obstet Gynecol Surv. 2009 Jan;64(1):50-7. doi: 10.1097/OGX.0b013e318193299b.


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