Ultrasound is a widely used and safe prenatal imaging technique that offers valuable insights into a developing fetus. Its primary medical purpose involves assessing fetal health, growth, and development throughout pregnancy. Beyond these health evaluations, ultrasound also plays a role in determining fetal gender. This offers an early glimpse into the baby’s characteristics.
When Gender Becomes Identifiable
The biological development of external genitalia allows for gender identification through ultrasound. While the internal sex organs begin to differentiate around 9 weeks of pregnancy, the external genitalia, such as the penis or clitoris and labia majora, start to differ around 11 weeks. At this early stage, the genital tubercle, a small bulge that will develop into either male or female genitalia, is present. The angle of this “nub” relative to the spine can offer an early indication of gender, with studies showing varying accuracy rates for this “nub theory” from 11 weeks onward.
For instance, at 11 weeks, the accuracy of predicting gender using the nub theory is around 70%, increasing to nearly 100% by 13 weeks, provided the fetus is in a suitable position for clear imaging. While some specialized centers might offer early gender determination as early as 14 weeks with high accuracy (up to 99%), the optimal window for more reliable determination is during the anatomy scan. This comprehensive scan occurs between 18 and 22 weeks of gestation, when the baby’s anatomy is sufficiently developed for clearer visualization and more accurate identification of gender.
Factors Affecting Visibility and Accuracy
Several elements can influence the ability to accurately determine gender during an ultrasound. Fetal position is a factor; if the baby is curled up, has legs crossed, or is facing away, the genital area may be obscured, making it difficult to get a clear view. The amount of amniotic fluid surrounding the fetus also plays a role, as adequate fluid levels can enhance imaging clarity. Conversely, too little fluid can make visualization challenging.
Maternal body habitus, such as a higher body mass index (BMI), can affect image clarity because ultrasound waves must travel through more tissue to reach the fetus, potentially attenuating the signal. Equipment quality and the sonographer’s experience and skill are also influential. Advanced ultrasound technology, including 3D and 4D imaging capabilities, can provide clearer views. An experienced sonographer is better equipped to identify subtle gender markers and navigate challenging imaging conditions, leading to more reliable results.
The Sonographer’s View
Sonographers are trained professionals who examine ultrasound images to determine fetal gender, focusing on distinct anatomical structures. For identifying male genitalia, they look for the “turtle sign,” where the penis and scrotum appear like a turtle’s head peeking out from behind its body.
The presence of a scrotal sac and the visible shape of the male genitalia are indicators. A “dome shape” between the legs, indicating the penis and scrotum, can also be observed.
For female genitalia, sonographers look for the “hamburger sign” or “three lines,” which represent the labia and clitoris. The labia may resemble a hamburger bun, with the clitoris appearing as the patty.
Both sagittal (side view) and transversal (cross-sectional) views are used to confirm findings and ensure a clear, unambiguous view before a determination is made. This ensures optimal positioning and clarity for the most accurate assessment.