What to Know About Ultrasound to Learn Baby's Sex

Medically Reviewed by Traci C. Johnson, MD on July 17, 2023
4 min read

If you’re pregnant and can't wait 9 months to find out your baby’s gender, there’s good news. There are many tests that can be used at different stages of your pregnancy to learn the gender of your baby. In this article, we’ll talk about a safe ultrasound test called the fetal ultrasound or sonogram. 

‌A fetal ultrasound is a test carried out during pregnancy that uses sound waves to create an image of your baby in the uterus. An ultrasound is a normal part of prenatal care in the U.S. because it can help your doctor find out important information about your baby’s growth and development. It can also help your doctor check for any problems your baby could be having. ‌

During an ultrasound, you will lie on an examination table and some gel will be spread over your abdomen.  A small device called a transducer will be moved over your skin and pressed against your belly, sending sound waves into your womb. Images are then collected and transferred to a screen. ‌

Once the test is over, the gel will be wiped clean and your doctor will discuss the results with you. You can get printed copies of the ultrasound to take away if you wish. Your doctor may sometimes ask for additional tests if he or she wants to collect more information about your baby's health. 

  • First trimester: Your first ultrasound takes place during the first trimester which is the time from the first day of your last period (usually before you’re pregnant) to 13 weeks. In the first ultrasound, your doctor will confirm whether you’re pregnant and also how long you’ve been pregnant. This may be done with a vaginal probe instead of an abdominal probe. You can also find out if you’re having more than one baby and your due date.
  • Second trimester: The next ultrasound is carried out in the second trimester (14 to 26 weeks) to find out how well your baby is growing and developing. . 
  • Third trimester: An additional ultrasound may be ordered for the third trimester (27 to 40 weeks). At this time, your doctor may want to find out whether your baby is growing as expected, the baby’s position, and the baby’s expected weight. 

Your doctor may also check the location of the placenta, and how much amniotic fluid is present around the baby. 

The placenta is a structure that develops during pregnancy to provide oxygen and nutrients to your baby as it grows. It also helps with removing waste products from the baby's blood. Amniotic fluid surrounds the baby inside the uterus and works like a shock absorber to keep the baby safe from outside pressures.

‌An ultrasound can be used to determine the baby’s gender towards the end of the first trimester and during the second trimester. Ahead of the test, tell your ultrasound doctor if you want to know the gender of your baby, or if you want to keep it as a surprise when the baby is born. ‌

The accuracy of determining your baby’s gender increases with how far along you are in the pregnancy. The accuracy can vary from 70.3% at 11 weeks to 98.7% at 12 weeks, and 100% at 13 weeks. Eleven weeks is the earliest that sex determination can be carried out with an ultrasound using a method called the ‘nub theory’.

‌All babies have a genital tubercle, informally called a ‘nub’ seen between the legs. The nub usually develops around 11 to 13 weeks. According to the nub theory, the baby is a male if the nub points to an angle greater than 30 degrees against the spine. The baby is a female if the nub is parallel to the spine, or at an angle less than 10 degrees against the spine. 

When the nub theory was tested on 656 singleton (one baby) pregnancies in a controlled study, it was possible to identify gender in 93% of the babies. 

Even though the accuracy rate is high from 12 weeks onwards, an ultrasound isn’t a 100% fool-proof method to identify the sex of your baby. But it's a low-risk procedure with no side effects. ‌

The fetal ultrasound is mostly carried out for medical reasons. While it can be helpful to find out your baby’s sex, and get pictures and videos to take home, these features should be seen as a secondary benefit. Therefore, if you are using an ultrasound to determine the baby's gender, it will usually not be done until the mid part of the second trimester.

‌Your doctor will give you advice on how to prepare for your ultrasound. You may be asked to drink up to 6 glasses of water before your test so your bladder fills up. This gives your doctor a better view of the baby on the ultrasound images. You may be asked not to urinate until after the test is over.  ‌

You may be asked to change to a hospital gown during the scanning process. The scanning is generally painless but let the technician know if you experience any signs of discomfort. Your ultrasound will usually be finished in about 30 minutes.