Learn more about the accuracy of gender ultrasounds, and find out how doctors determine if you’re having a boy of girl. 

Ultrasounds are a key tool for analyzing the health of your fetus, but they also serve another exciting purpose: Revealing whether you're having a boy or girl. But how do gender ultrasounds work, and are they always accurate? We spoke with experts to break down your most pressing questions.

How Do Gender Ultrasounds Work?

While you're reclining on an exam table, the ultrasound technician will slather gel on your belly, then she'll glide over it with a plastic transducer that emits high-frequency sound waves. These waves bounce off of your little one's body to produce an image of her soft tissues, organs, and other anatomy—including reproductive parts. The image will be displayed on a monitor for you and your partner to see. 

Ultrasounds are considered safe, and they don't involve X-rays or radiation. The exam also doesn't hurt, although the gel may feel cold and be messy. 

pink and blue shoes and ultrasound
Credit: Getty Images

Boy vs. Girl Ultrasound Predictions: Are They Accurate?

As it turns out, gender ultrasounds are pretty accurate. One recent study found that the ultrasound technician correctly predicted a baby's gender 98 percent of the time. Still, the results of your individual exam will depend on a number of factors, including timing, your baby's position, your body size, and whether you're carrying multiples. 

Timing: "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.

Your Baby's Position: "Some fetuses are bashful and keep their legs together and impair determination of gender," says Dr. Putterman. "If they are not clearly visualized, mistakes can be made when sonographers guess the gender based on a suboptimal examination." If your sonographer is having a tough time seeing between your baby's legs, take any answer you get from her with a grain of salt.

Your Body Size: If you're plus-sized, that additional body mass can prohibit a clear image of your baby. "Determining a fetus' sex on ultrasound is certainly more difficult in women who are overweight or obese," says Michele Hakakha, M.D., a Beverly Hills-based OB-GYN and the author of Expecting 411. 

Pregnancy with Multiples: If you're carrying twins or triplets (or more!), your babies could hide their siblings, making 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, according to some experts. "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 your later ultrasounds, such as your 20-week anatomy scan, for a more definitive answer.  Bottom line: Don't go out and buy that pretty pink dress after your very first ultrasound, unless you keep the receipt!