Ultrasound technology offers a non-invasive method to determine a baby’s biological sex. This imaging technique uses sound waves to create visual representations of the fetus. While primarily for monitoring fetal health, ultrasounds also allow observation of external genitalia.
When Gender is Visible
A baby’s sex can potentially be determined by ultrasound around 12 weeks of gestation, though this is challenging. External genitalia are not fully developed until approximately 13 weeks. For greater certainty, experts recommend waiting until 18 to 22 weeks of pregnancy, which coincides with the routine anatomy scan. While determination between 11 and 14 weeks might be 70-75% accurate, accuracy approaches nearly 100% after 14 weeks, especially by 18 to 20 weeks.
What Sonographers Look For
Sonographers examine specific anatomical markers to determine a baby’s biological sex during an ultrasound. For male fetuses, the presence of male genitalia, including the penis and scrotum, is a clear indicator. Sonographers often look for what is termed the “turtle sign,” which describes the appearance of the penis tip emerging from behind the testicles. Another sign for boys is the “dome shape,” visible when looking at the baby from below, indicating the penis and scrotum.
For female fetuses, sonographers identify the appearance of the labia and clitoris. This is commonly referred to as the “three-line sign” or “hamburger sign.” The three lines represent the labia majora, labia minora, and the clitoris, resembling a hamburger bun with a patty in between.
An earlier, less definitive method is the “nub theory,” which can be considered between 11 and 14 weeks of gestation. This theory involves observing the angle of the developing genital tubercle, a small protrusion that will eventually form either a penis or a clitoris, relative to the baby’s spine. A tubercle angled upwards by more than 30 degrees from the spine suggests a male, while a flatter or downward-pointing angle indicates a female. However, the “turtle” and “three-line” signs, visible later in pregnancy, offer more definitive identification.
How Accuracy is Determined
Several factors influence the accuracy of gender determination via ultrasound. The baby’s position within the womb is a primary consideration, as crossed legs, a curled-up posture, or facing away from the transducer can obscure the genital area. The umbilical cord or even the baby’s fingers can sometimes be mistaken for genitalia, leading to an incorrect assessment.
The gestational age at which the scan is performed also plays a role in accuracy. Ultrasounds conducted too early, before the external genitalia are sufficiently developed, can result in less reliable predictions. Conversely, scans performed later in pregnancy, typically between 18 and 22 weeks, offer higher accuracy rates due to the baby’s increased development.
The skill and experience of the sonographer, along with the quality of the ultrasound equipment, contribute to the clarity and interpretability of the images. An experienced sonographer is better equipped to navigate challenging views and accurately identify anatomical structures. Maternal factors, such as body habitus, can also affect image clarity, as sound waves may be more challenging to penetrate through certain tissue densities.
Overall, mid-pregnancy ultrasounds are highly accurate for determining biological sex, with rates often ranging from 95% to 99% when conditions are optimal. However, it is important to understand that no method is entirely without a margin of error. While genetic testing methods can offer nearly 100% accuracy earlier in pregnancy, ultrasound remains a widely used and reliable tool for many expecting parents.