Ultrasound provides a non-invasive method for monitoring fetal growth and development throughout pregnancy. It uses high-frequency sound waves to create live images of the fetus, placenta, and uterus, allowing healthcare providers to assess health and structural development. Determining the sex of the developing fetus is a common inquiry, but the ability of ultrasound to provide this answer depends heavily on the timing of the scan and the visibility of specific anatomical markers.
Optimal Timing for Fetal Sex Identification
Determining fetal sex with ultrasound is a process highly dependent on the stage of gestation, as the external genitalia undergo a period of differentiation. The standard time for a definitive determination occurs during the routine second-trimester anatomy scan, typically scheduled between 18 and 22 weeks of pregnancy. At this stage, the accuracy of sonographic sex determination is exceptionally high, often falling between 95 and 99% under favorable conditions.
Attempts to identify the sex can be made earlier, but with a noticeable reduction in reliability because the external structures are not yet fully formed. Around 11 to 14 weeks of gestation, the external genitalia emerge from a structure called the genital tubercle, which initially appears similar in size and shape. Before 12 weeks, the overall success rate for correct sex assignment is significantly lower, sometimes around 75%.
Accuracy rapidly increases as the pregnancy progresses. Waiting until the mid-second trimester ensures that the developing structures have completed their crucial period of differentiation and are large enough for clear visualization.
Anatomical Markers Used in Ultrasound
The sonographer relies on identifying specific visual signs that change depending on the fetus’s gestational age.
Early Markers (11–14 Weeks)
In the earlier weeks (11 to 14 weeks), the primary method involves assessing the angle of the genital tubercle, often referred to as the sagittal sign. This assessment is performed in a mid-sagittal view, where the fetus is seen in profile. If the genital tubercle projects upward at an angle greater than 30 degrees relative to the lumbosacral skin line (the line of the fetal spine), it indicates developing male genitalia. Conversely, if the tubercle is parallel to the spine or angled downward at less than 10 degrees, it suggests female genitalia. If the angle falls into an indeterminate range between 10 and 30 degrees, a definitive conclusion cannot be made.
Second Trimester Markers (18+ Weeks)
Later in the second trimester, the external structures are more distinct, allowing for direct visualization. For a male fetus, the appearance is described using the “turtle sign,” which refers to the visualization of the penis and the scrotum. The penis is seen as an elongated structure protruding outward, and the scrotum appears as a distinct sac. For a female fetus, the developing labia and clitoris are described using the “hamburger sign.” This term refers to the three distinct echogenic (bright) lines visible in the transverse view of the perineum. These three lines represent the labia majora and the clitoris, allowing for confident assignment of the female sex.
Factors Affecting the Accuracy of the Reading
Even within the optimal gestational window, several variables external to the fetal anatomy can prevent a clear image and affect the reliability of the reading.
Fetal Position and Obstruction
One of the most common challenges is the position of the fetus during the examination. If the fetus is curled tightly, has its legs crossed, or is turned away from the transducer, the perineal area may be completely obscured. The placement of the umbilical cord or a limb between the legs can also be mistaken for external genitalia, leading to an incorrect determination. If the view is suboptimal, the sonographer cannot confidently identify the anatomical structures and will either report an inconclusive result or recommend a repeat scan.
Maternal and Technical Factors
Maternal physical factors, particularly a higher body mass index (BMI), can reduce the quality of the ultrasound image. Increased maternal tissue attenuates, or weakens, the sound waves as they travel from the transducer to the fetus and back, which results in a less clear picture. Technical and human elements also play a part in the accuracy of the reading. The quality of the ultrasound equipment and the experience and training level of the sonographer contribute to the ability to obtain a diagnostic image and correctly interpret subtle findings.