How Accurate Are Gender Ultrasounds?

Ultrasounds are a key tool for monitoring the health of your fetus, but they can also serve another exciting purpose for some parents: Revealing their sex. But how do ultrasounds work to predict gender, and are they always accurate? We spoke with experts to break down your most pressing questions.

While this article uses the terms "gender" and "boy" vs. "girl," it's important to note that gender is a personal identity that exists on a spectrum, can change over the course of a person's lifetime—and most importantly—is something that a person defines for themselves. Sex, on the other hand, is assigned at birth based on the appearance of a baby's genitalia. While sex assigned at birth often matches a person's gender (called cisgender), sometimes, for transgender, intersex, and gender nonbinary people, it does not.

What Is a Gender Ultrasound?

Health care providers do not perform ultrasounds during pregnancy solely to determine your baby's sex. Instead, technicians observe your baby's genitalia as part of a routine anatomy scan, which takes place in the second trimester, usually between 18 and 22 weeks.

While some people wait until their baby is born to find out if they're having a boy or a girl, others want to know as soon as possible. Ultrasounds are a non-invasive way to get a good guess during pregnancy.

These ultrasounds, though commonly referred to as "gender ultrasounds," are actually anatomy scans that evaluate how your baby is developing. Technicians always look at genitalia to evaluate whether there's 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).

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.

According to Penn Medicine, during an anatomy scan, a health care provider will evaluate the fetus from head to toe, including a detailed look at:

• Their estimated weight and length
• Their face, paying particular attention to lips and palate
• Their brain
• Their spine
• Their heart
• Their kidneys
• Their limbs

In addition, a technician will assess the health of the placenta. Technicians will also view your baby's genitalia. The image will be displayed on a monitor for you (and anyone you may have with you) to see, if you'd like.

According to the Food and Drug Administration (FDA), ultrasounds are considered safe when done by a trained technician for medical purposes, and they don't involve X-rays or radiation. The exam also doesn't hurt, although the gel may feel a bit cold and messy.

Accuracy of Boy vs. Girl 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. A 2015 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 based on ultrasound during the first trimester (between 11 and 14 weeks in the study) are less accurate with ultrasound technicians being right about 75% of the time.

A 2008 study published in peer-reviewed journal Acta Obstetricia et Gynecologica Scandinavica had similar results: "The success rate for correctly identifying fetal gender (where identification was possible) increased with gestational age, from 71.9% at 11 weeks, 92% at 12 weeks, and 98.3% at 13 weeks, respectively, where gestational age was calculated from the crown-rump length in conjunction with menstrual or ovulation dating," according to researchers.

Typically, you can expect an ultrasound between 18 and 22 weeks during which the technician is likely to share their findings unless you explicitly ask them not to. 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, M.D., 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 your baby'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, M.D., 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.

When Should I Get a Gender 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.

Note, however, that parents have the option for non-invasive prenatal screening (NIPT), which 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. While it's not covered by insurance for all patients, many companies will negotiate an out-of-pocket cost that is very reasonable, and possibly on par with the for-profit ultrasounds offered at many "for fun" ultrasound locations. NIPT can usually be done around 10 weeks of pregnancy or later.

The Bottom Line

It's exciting to learn your baby's sex, but keep in mind that these predictions are sometimes inaccurate or incomplete. In some cases, technicians make an error. And in other cases, your child's gender (their internal sense of who they are) may not match the sex they were assigned at birth. So, either way, it's best to keep an open mind.