How to Determine a Baby's Gender from an Ultrasound

Ultrasounds are a key tool for monitoring the health of your fetus, but they can also serve another exciting purpose: An ultrasound might be able to reveal whether you're having a boy or a girl.

While some people wait until their baby is born to find out their biological sex, others want to know as soon as possible. One study found that 69% of respondents planned to learn the sex of their fetus.

But how do ultrasounds work to predict gender, and are they always accurate? We spoke with experts to answer your most pressing questions.

What Is a Gender Ultrasound?

A prenatal ultrasound is a non-invasive test that uses sound waves to produce images of a fetus. It's a preferred method of imaging during pregnancy as it doesn't involve radiation or pose harm to either the fetus or the pregnant person. The Food and Drug Administration (FDA) says ultrasounds are considered safe when done by a trained technician for medical purposes.

Between 18 and 22 weeks, you'll get a detailed anatomy scan, called a level 2 ultrasound. Though sometimes called "gender ultrasounds," anatomy scans aren't solely to determine your baby's sex. Rather, they evaluate how the fetus is developing in the womb.

What Happens During the Ultrasound

While you're reclining on an exam table, the ultrasound technician will apply gel on your belly. Then, they'll glide over it with a plastic transducer that emits high-frequency sound waves. These waves bounce off your little one's body to produce an image of their soft tissues and organs.

During the anatomy scan, the technician will do the following:

• Measure the size of your baby
• Evaluate the fetus from head to toe
• Listen to the heart rate
• Check the major organs
• Determine the level of amniotic fluid 
• Evaluate the position of the placenta
• Look at the baby's sex organs

The technician will always look at genitalia to evaluate whether there are any problems, but they don't need to tell the parents about their baby's sex (and if there's an issue, the physician can divulge as much information as the parents want to hear).

Accuracy of Gender Ultrasound Predictions

As it turns out, ultrasounds are pretty accurate for identifying whether a fetus has a penis or a vulva, particularly in the second and third trimesters.

One study found that ultrasound technicians correctly predict a baby's assigned sex nearly 100% of the time after 14 weeks gestation. Estimates regarding a fetus' sex between 11 and 14 weeks were less accurate, with ultrasound technicians being right about 75% of the time. The most common mistake was to assign male fetuses as female.

Other researchers have found similar results. Still, the accuracy of your exam results will depend on several factors, including timing, your baby's position, your body size, and whether you're carrying multiples.

Timing

How far along the pregnancy is makes a big difference in the accuracy of so-called gender ultrasounds. "The accuracy of the ultrasounds increases as gestational age advances," says Bart Putterman, MD, an OB-GYN at Texas Children's Pavilion for Women in Houston. Waiting until the second trimester increases the likelihood of getting an accurate reading because the fetus' genitals are more developed and easier to see.

Your baby's position

If your sonographer is having a tough time seeing between the fetus's legs because of their position, it will impair their ability to determine your baby's sex. "If they are not clearly visualized, mistakes can be made when sonographers guess the gender based on a suboptimal examination," says Dr. Putterman.

Your body size

If you have a larger body, determining a fetus' sex on ultrasound is more difficult, says Michele Hakakha, MD, a Beverly Hills-based OB-GYN and the author of Expecting 411: An Insiders Guide to Pregnancy and Childbirth. However, an experienced ultrasound operator should be able to get a clear picture for people of all sizes.

Pregnancy with multiples

If you're carrying twins or triplets (or more!), your babies could hide their siblings, making the determination of the sex of each baby more difficult.

Ultrasound Signs for Boys and Girls

When determining the sex of the fetus on ultrasound, the technician will look for characteristic features known as signs. 

Girl Ultrasound Signs

Some new parents automatically think the absence of a penis indicates a girl, but this isn't necessarily true. In fact, particularly early on in pregnancy, a clitoris and a penis are roughly the same size and shape. Here are some indicators that you're expecting a girl:

• Hamburger sign: During the ultrasound, the technician will look for the appearance of female genitalia—the labia and clitoris. If you look closely at the image, you'll see that the labia lips look similar to a hamburger bun, while the clitoris resembles a hamburger patty.
  
• Sagittal sign: Each sex has a sagittal sign. It's 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, then the fetus is likely 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:

• Male genitalia: The presence of male genitalia (including testicles, scrotum, and penis) is a clear sign of male sex. The ultrasound technician may look for the "turtle sign"— the tip of the penis peeking out from behind the testicles.
• Sagittal sign: If the caudal notch is pointing upward, then the fetus is likely a boy. If it's somewhere in between, it may be harder to make a definitive determination.

When Can You Find Out Gender With an Ultrasound?

Early ultrasounds aren't the most accurate gauge of whether you're expecting a boy or girl.

"The earliest in pregnancy that the fetus's sex can be determined by ultrasound is about 12 weeks, and even then, it can be very difficult," says Dr. Hakakha. "The external genitalia—the vulva or penis and scrotum—are not actually external until about 13 weeks."

You may want to wait until later ultrasounds, such as your 20-week anatomy scan, for a more definitive answer.

Other Ways to Determine Sex

In addition to an ultrasound, the sex of your baby can be determined a few other ways, including the following.

Non-invasive prenatal screening (NIPT): NIPT is a blood test that scans for genetic abnormalities, including Down Syndrome, trisomy 18, and trisomy 13. It can also determine sex by chromosomal analysis. NIPT can usually be done around 10 weeks of pregnancy or later.

Chorionic villus sampling (CVS): Conducted between 10 and 13 weeks of pregnancy, CVS involves taking cells from the placenta to screen for chromosomal or genetic abnormalities. You can determine your baby's sex as a byproduct of the procedure.

Amniocentesis: You can also find out your baby's sex through amniocentesis, a procedure in which fluid is extracted from the sac surrounding your baby. Amniocentesis is generally conducted between 15 and 20 weeks of pregnancy, and it can diagnose birth defects or genetic conditions.

Additional reporting by Robin Elise Weiss, PhD, MPH