Ultrasound is the most common way to find out a baby’s sex before birth. The technician looks for specific genital markers on the image. While not 100% guaranteed, a clear view of these markers is highly accurate after a certain point in pregnancy. Most parents learn the sex at their mid-pregnancy scan, which usually happens between 18 and 22 weeks. The accuracy depends heavily on the baby’s position, the timing of the scan, and the skill of the person reading the image.
How Does an Ultrasound Show the Sex?
Ultrasound uses sound waves to create a picture of the baby inside the womb. The sound waves bounce off the baby’s tissues and bones, creating a live image on a screen. The image is black, white, and gray, which can make it hard for untrained eyes to see the details.
For a boy, the technician looks for the penis and scrotum. For a girl, the technician looks for the labia and clitoris. In a good image, these structures have distinct shapes that differ clearly from each other. The angle of the baby’s body matters. The technician needs a clear view between the baby’s legs to make a confident call.
When Is the Best Time to Find Out?
The most reliable time for a sex determination ultrasound is between 18 and 22 weeks of pregnancy. This is typically the same scan used to check the baby’s anatomy and development. By this point, the external genitalia have formed enough to be visible on the screen.
Some clinics offer early sex determination scans between 11 and 14 weeks. At this stage, the genitals are still developing, and the difference between a boy and a girl is less obvious. The accuracy at this early stage is lower, and some clinics do not offer sex determination this early because of the higher error rate. If you have an early scan, treat the result as a guess rather than a certainty.
What Does a Boy Look Like on Ultrasound?
When the technician identifies a boy, they look for specific shapes. The penis appears as a small, pointed projection extending outward from the body. The scrotum appears as a rounded, slightly darker area behind the penis.
A common technique is the “sagittal sign.” This looks at the angle of the genital tubercle, which is the tissue that becomes the penis or clitoris. In a boy, this tubercle points upward at an angle greater than 30 degrees from the spine. This sign can be used early in pregnancy, but it is not always reliable.
What Does a Girl Look Like on Ultrasound?
For a girl, the technician looks for the labia, which appear as three or four parallel lines in the genital area. This is sometimes called the “hamburger sign” because the labia resemble a hamburger bun with the clitoris in the middle.
In the sagittal sign, a girl’s genital tubercle points downward or horizontally, at an angle less than 30 degrees from the spine. The absence of a penis is not enough to confirm a girl. The technician must see the female structures clearly to make a positive identification.
How Accurate Is a Sex Determination Ultrasound?
When performed after 18 weeks by an experienced technician, sex determination ultrasounds are very accurate. Studies consistently show accuracy rates above 95 percent for scans done in the second trimester. The accuracy is highest when the baby is in a good position and the technician gets a clear image.
Errors do happen. The most common cause of error is the baby’s position. If the baby is facing the wrong way, has its legs crossed, or has the umbilical cord between its legs, the image can be misleading. The umbilical cord can sometimes be mistaken for a penis, leading to a wrong “boy” result. A girl can be mistaken for a boy if the labia are swollen, which can happen from maternal hormones.
It is also important to note that ultrasound is a medical imaging tool, not a guarantee. The primary purpose of the mid-pregnancy scan is to check the baby’s health and development. Sex determination is a secondary benefit, not the main medical goal.
What Can Make the Scan Less Accurate?
Several factors can reduce the accuracy of a sex determination ultrasound. The baby’s position is the biggest factor. If the baby is breech, meaning feet first, or lying face down, the genital area may be hard to see. The technician may ask you to change positions, walk around, or drink water to encourage the baby to move.
Maternal body habitus can also affect image quality. Extra abdominal tissue can make the ultrasound waves travel farther, reducing the clarity of the image. The amount of amniotic fluid matters too. Too little fluid can make it hard to get a clear view, while too much can make the baby move around excessively.
Multiple pregnancies add another layer of complexity. In twins, the technician must identify each baby separately and determine the sex of each one. This can be more challenging, especially if the babies are positioned close together.
Are There Other Ways to Find Out the Sex?
Ultrasound is not the only method for determining fetal sex. Noninvasive prenatal testing, or NIPT, is a blood test that screens for chromosomal abnormalities. This test can also reveal the sex by looking for fragments of the baby’s DNA in the mother’s blood. NIPT is highly accurate for sex determination, with accuracy rates above 99 percent. It is typically done after 10 weeks of pregnancy.
Amniocentesis and chorionic villus sampling, or CVS, are invasive tests that can determine sex with near-perfect accuracy. These tests are not done for sex determination alone because they carry a small risk of miscarriage. They are performed when there is a medical reason to check the baby’s chromosomes.
Home tests that claim to determine sex from a urine sample are not regulated by the FDA and have not been validated in rigorous clinical trials. Some research suggests these tests have some accuracy, but the evidence is not strong enough to recommend them. If you want to know the sex before the mid-pregnancy scan, NIPT is the most reliable non-invasive option.
What If the Technician Cannot Tell?
Sometimes the technician simply cannot get a clear view. This is common and not a cause for concern. The baby may be in a position that hides the genital area. The technician may ask you to come back for another scan in a few weeks, when the baby has likely changed position.
Some parents choose not to find out the sex at all. This is a personal choice with no right or wrong answer. If you do want to know, let your technician know at the start of the scan. They will try to get the best view possible, but they cannot guarantee a result at every appointment.
Frequently Asked Questions
How early can you find out the sex on an ultrasound?
Most clinics offer sex determination at the mid-pregnancy scan, between 18 and 22 weeks. Some clinics attempt it as early as 11 to 14 weeks, but accuracy is significantly lower at that stage.
Can the ultrasound be wrong about the sex?
Yes, but errors are rare after 18 weeks, with accuracy rates above 95 percent. Mistakes are usually caused by the baby’s position or the umbilical cord being mistaken for a penis.
Is a blood test more accurate than an ultrasound for sex?
Yes, noninvasive prenatal testing (NIPT) is more than 99 percent accurate for sex determination. It is a blood test done after 10 weeks and is also used to screen for chromosomal conditions.
What does the hamburger sign mean on an ultrasound?
The hamburger sign is the appearance of a girl’s labia and clitoris on an ultrasound image. The labia look like the bun, and the clitoris sits in the middle, resembling a hamburger.
