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 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.

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Everything to Know About Ultrasounds

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

Prenatal Ultrasound

A prenatal ultrasound is a non-invasive test that uses audible sound waves to produce images of a fetus's shape and position in the uterus. 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 trained sonographer will perform a detailed anatomy scan called a level 2 ultrasound. 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. While telling the difference can sometimes be tricky, there are a few things the sonographer will look for to make the determination.

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% 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 sex was 75%. 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:

  • Hamburger sign: This 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.
  • Sagittal sign: Each sex has a sagittal sign. It is obtained by looking at a profile view of the fetus (known as the midline sagittal plane). There is a nub at the end of the spine, called the caudal notch. If it is pointing downward at a 10-degree angle, then the fetus is a girl.

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 a baby boy would be based on the following signs:

  • Sagittal sign: If the caudal notch is pointing upward at more than a 30-degree angle, then the fetus is a boy. If it is somewhere in between, it may be harder to make a definitive determination.
  • Flow of urine: The flow of urine can sometimes be spotted in a fetus. If it is moving upward, then it is more likely a boy.
  • Male genitalia: Often be seen by weeks 18 to 20, the presence of male genitalia, including testicles, scrotum, and penis, 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 is 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% of parents preferred to know the baby's sex after an amniocentesis compared to 28% who preferred an ultrasound. This was despite the fact that 96.2% of women believed that an ultrasound was a reliable means of determining the sex of her baby.

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  1. Kooper AJ, Pieters JJ, Eggink AJ, et al. Why do parents prefer to know the fetal sex as part of invasive prenatal testing?. ISRN Obstet Gynecol. 2012;2012:524537. doi:10.5402/2012/524537

  2. 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

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

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