Telling a boy from a girl on an ultrasound comes down to a few specific visual clues, and which clues you’re looking for depends on how far along the pregnancy is. Before about 14 weeks, boys and girls look almost identical on screen because the external genitalia haven’t fully differentiated yet. The most reliable window for sex identification is the mid-pregnancy anatomy scan, typically done between 18 and 22 weeks, when the genitalia are developed enough to see clearly.
What to Look for on a Boy Ultrasound
On a second-trimester ultrasound, a male fetus is identified by the visible presence of a penis, scrotum, and sometimes the testicles. In a clear image, the penis often appears as a small protrusion between the legs, sometimes called the “turtle sign” because the rounded tip of the penis can peek out from behind the scrotum in a way that resembles a turtle’s head. Sonographers also look for the scrotal sac, which appears as a rounded structure below the penis.
The raphe, a faint line running along the middle of the scrotum, can sometimes be seen on high-resolution images and provides additional confirmation. When you’re looking at a printed ultrasound image, a boy typically shows an obvious protrusion in the genital area when viewed from between the legs or in profile.
What to Look for on a Girl Ultrasound
Female genitalia on ultrasound produce what’s commonly called the “hamburger sign” or the “three-line sign.” This is exactly what it sounds like: three parallel lines visible between the legs, representing the two outer labia (labia majora) with the clitoris or inner labia between them. The lines are roughly parallel and straight in a normal female fetus.
This sign is best seen when the baby’s legs are apart and the ultrasound beam is aimed directly at the genital area from below. Because female anatomy sits closer to the body and doesn’t protrude, it can be harder to identify in a less-than-ideal image. The absence of a visible penis alone is not enough to confirm a girl, which is why sonographers specifically look for those three lines as a positive sign.
The Nub Theory for Early Scans
If you’re trying to guess the sex from a first-trimester scan (around 11 to 13 weeks), the only method with real scientific backing is called the “nub theory.” At this stage, both boys and girls have a small genital tubercle, a tiny bud of tissue between the legs that eventually develops into either a penis or a clitoris. The theory relies on measuring the angle of this nub relative to the baby’s spine.
In a side-view (sagittal) image, a genital tubercle angling upward at more than 30 degrees from the lower spine suggests a boy. A tubercle that lies flat or angles slightly downward (less than 10 degrees from horizontal) suggests a girl. When the angle falls between 10 and 30 degrees, the sex can’t be reliably determined. This method works best after 12 weeks, and even then it requires a very specific image angle that your printed ultrasound may not show clearly.
Related to this, sonographers sometimes reference the “sagittal sign.” In a midline side view of the genital area, a small notch toward the baby’s head (cranial notch) indicates male, while a notch toward the baby’s tailbone (caudal notch) indicates female. Both of these methods require a trained eye and precise imaging, so they’re difficult to apply to the grainy printouts you take home.
Why Some Ultrasounds Are Inconclusive
Several factors can make it impossible to determine sex, even at the 20-week scan. The most common reason is fetal position. If the baby’s legs are crossed, pressed together, or the baby is facing away from the ultrasound probe, the genital area simply isn’t visible. Babies move constantly during scans, and sometimes they just don’t cooperate.
The umbilical cord is another common source of confusion. A loop of cord between the legs can look like a penis to an untrained eye, which is one reason sonographers trace the cord’s path carefully before making a call. On a still image you take home, it can be genuinely hard to tell cord from anatomy without the context a live scan provides.
Maternal body composition also plays a role. More tissue between the ultrasound probe and the baby means the image loses clarity, which can make fine details like the three-line sign or the scrotal raphe harder to resolve. Low amniotic fluid around the baby’s lower body creates the same problem by reducing contrast.
Does the Ramzi Theory Work?
You may have seen claims online that the placenta’s position at 6 to 8 weeks can predict sex: placenta on the right means boy, placenta on the left means girl. This is called the Ramzi theory, and it is not supported by reliable evidence. The original study behind it was never published in a peer-reviewed journal, and no major medical organization recognizes it. A 2010 study that attempted to replicate the approach using similar methods in 277 pregnancies found no relationship between placental location and fetal sex.
If you’re eager to know the sex before your anatomy scan, a blood-based screening (cell-free DNA testing) performed as early as 10 weeks is far more accurate than any ultrasound-based guessing method at that stage.
When Accuracy Is Highest
At 11 to 12 weeks, sex determination using the genital tubercle angle is possible but limited. The nub is small, the required imaging angle is very specific, and intermediate angles leave many cases undetermined. Accuracy improves significantly with each passing week as the external genitalia develop.
By 18 to 22 weeks, trained sonographers identify fetal sex with high reliability when the baby is in a favorable position. At this stage, the anatomy is distinct enough that sonographers are looking for the specific structures themselves (penis, scrotum, labia) rather than indirect angle measurements. If your sonographer tells you they couldn’t determine the sex at this scan, it’s almost always a positioning issue rather than ambiguous anatomy. A follow-up scan or a simple change in the baby’s position during the same appointment often resolves it.
Reading Your Own Ultrasound Printout
When you look at your take-home ultrasound image, keep a few things in mind. First, identify the view. A between-the-legs view (looking up from below the baby) is the most useful for sex determination. In this view, a boy shows a clear protrusion, and a girl shows three parallel lines. A side-view profile can show the nub angle in early pregnancy or the penis in later pregnancy, but it’s harder to interpret without training.
Second, look for labels. Many sonographers mark the genital area on the image or include an arrow. Some write the sex directly on the printout or place it in a sealed envelope if you’ve asked for a reveal.
Third, be cautious with early images. Before 14 weeks, the genital tubercle looks similar in both sexes. Online gender-prediction groups often interpret these images with great confidence, but the reality is that trained radiologists still find many early cases indeterminate. A confident answer from your sonographer at 20 weeks is worth more than any amount of squinting at a 12-week printout.