How to Tell a Baby's Gender on the Ultrasound

Pregnancy Ultra-Sound
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Most parents today will want to find out the sex of their baby before the birth. One of the most common ways to do this is with an ultrasound, most frequently performed at between 18 and 20 weeks of gestation.


What's It Like Seeing Your Baby in an Ultrasound?

According to a 2012 study published in the journal Obstetrics and Gynecology, no less than 69 percent of parents wanted to know. Among the reasons cited, 77.8 percent wanted to know "out of curiosity," 68 percent "just wanted to know," and 66.8 percent did so "because it was possible."

Ultrasound in Prenatal Care

Ultrasound is a non-invasive technology which uses audible sound waves to create high-contrast images. It is commonly used to visualize fetuses during prenatal care, and, when used in this way, is referred to as obstetric sonography. It is a preferred method of imaging during pregnancy as it neither involves radiation nor poses harm to either the fetus or mother.

An ultrasound is routinely used at different stages of the pregnancy. While most practitioners will wait until at least six weeks to perform the first ultrasound, the gestational sac may be seen as early as four and a half weeks, while a heartbeat may be detected as early as five.

Between weeks 18 and 20, a detailed anatomy scan called a level 2 ultrasound will be performed by a trained sonographer in a specialized clinic where the equipment will be more sophisticated. It is at this time that the sonographer will measure the size of your baby, check the major organs, measure the level of amniotic fluid to make sure that it's right, and check the position of the placenta.

By this stage of fetal development, you should also be able to find out the sex of your baby's. While telling the difference can sometimes be tricky, there are a few things the sonographer will look for to make the determination.

Ultrasound Accuracy

When a sonographer does a level 2 exam, what is seen on ultrasound is far more important than what is not seen. With regards to determining sex, what this means is that the absence of a penis does not inherently mean that you have a girl.

With that being said, over 99 percent of ultrasounds performed between weeks 18 and 20 will make the correct determination. It is only when it is performed before week 14 that the rate of accuracy can drop significantly.

According to a 2014 study from Australia, which reviewed 642 fetal ultrasound results performed between weeks 11 and 14, the overall success rate in determining fetal gender was 75 percent. The most common mistake was to assign male fetuses as female.

Girl Ultrasound Signs

When determining the sex of the fetus on ultrasound, the sonographer will look for characteristic features known as signs. For girls, there are two signs to look for:

  • The hamburger sign is the moniker given to the appearance of the labia and clitoris on an ultrasound. If you look closely at the image, you will see that the labia lips would look similar to a hamburger bun, while the clitoris would resemble the hamburger patty.
  • The sagittal sign is a bit more complex but involves taking a profile view of the fetus (known as the sagittal plane). If you follow the baby's spine to the tailbone, you will see a nub at the very end. If there is a downward-pointing notch (called the caudal notch), then the fetus would be a girl. At 14 weeks, the sagittal sign is 90 percent accurate in making the correct determination. By 18 weeks, the figure would be closer to 100 percent.

Boy Ultrasound Signs

You would think that male fetuses would be easier to identify than females, but that's not always the case. This is especially true before week 14. By weeks 18 to 20, the determination for male gender would be based on the following signs:

  1. The sagittal sign in a male fetus differs from a female in that the position of the caudal notch would be upward facing. In a female, the notch would point downward in a 10-degree position. In a male, it would be more in a 30-degree position. If it is somewhere in between, it may be harder to make a definitive determination.
  2. The flow of urine can sometimes be spotted in a fetus. If it is moving upward, then it is more likely a boy.
  3. The male genitalia, including testicles, scrotum, and penis, can often be seen by weeks 18 to 20 and is the irrefutable sign of male gender.

Other Ways to Determine Sex

In addition to an ultrasound, the sex of your baby can be confirmed with an amniocentesis or chorionic villus sampling (CVS). Amniocentesis a procedure in which fluid is extracted from the sac surrounding your baby with a needle and syringe. CVS involves taking cells from the placenta with a needle.

While both procedures carry risks, they are extremely accurate in making the determination and can return a result by as early as week 11 (for CVS) and week 15 (for amniocentesis).

In fact, the same study published in Obstetrics and Gynecology concluded that 65 percent of parents preferred to know the baby's sex after an amniocentesis compared to 28 percent who preferred an ultrasound. This was despite the fact that 96.2 percent of women believed that an ultrasound was a reliable means of determining the sex of her baby.

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  2. Di Muzio B, Gaillard F, et al. Early Pregnancy Radiographic Features: Antenatal Ultrasound. Radiopaedia. Published online, no date.

  3. Reddy UM, Abuhamad AZ, Levine D, Saade GR. Fetal imaging: executive summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging Workshop. J Ultrasound Med. 2014;33(5):745-57. doi:10.7863/ultra.33.5.745

  4. Kearin M, Pollard K, Garbett I. Accuracy of sonographic fetal gender determination: predictions made by sonographers during routine obstetric ultrasound scans. Australas J Ultrasound Med. 2014;17(3):125-130. doi:10.1002/j.2205-0140.2014.tb00028.x

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