A fetal ultrasound uses sound waves to create images of a baby inside the womb, allowing parents and clinicians to visualize the developing fetus, including the external genitalia. While the technology is highly reliable, determining the baby’s sex is not entirely guaranteed and carries a small margin of error. Several biological, technical, and situational factors can affect the sonographer’s ability to get an accurate reading.
General Accuracy and Optimal Timing
The accuracy of fetal sex determination using ultrasound depends directly on the timing of the scan. Under ideal circumstances, the procedure is highly accurate, often cited as having a success rate between 95% and 99% when performed at the correct gestational age. The optimal window for a reliable determination is typically during the anatomy scan, which is usually scheduled between 18 and 22 weeks of pregnancy.
Before this mid-pregnancy mark, accuracy significantly decreases because the fetal genitalia are still in the early stages of development and look very similar in both sexes. Attempts to determine sex earlier than 13 weeks can result in success rates as low as 54% to 70%. Waiting until the 18 to 22-week period allows for greater differentiation of the external structures, providing the sonographer with a clearer image.
Common Causes of Gender Misidentification
Misidentification often stems from mistaking a non-genital structure for a penis or failing to correctly identify the female anatomy. A frequent source of confusion is the umbilical cord, which can hang between the baby’s legs and appear as a protruding male organ on the image. Similarly, a fetal limb, such as a finger or toe, can be misinterpreted as male genitalia if positioned poorly during the scan.
In a female fetus, the labia can sometimes be swollen, especially in the second trimester, and this appearance may be mistaken for a scrotum. Sonographers look for the “three-line sign” to identify a girl, but an unclear image or swollen labia can mimic the appearance of male parts. Conversely, if a male fetus has its genitalia folded or tucked close to the body, it can be overlooked or mistaken for female anatomy.
In rare cases, misidentification can be due to congenital adrenal hyperplasia (CAH). This disorder causes a genetically female fetus to have external genitalia that appear more male, a condition known as ambiguous genitalia. This developmental difference presents an anatomical challenge to the sonographer, leading to an inconclusive or incorrect sex assignment.
External Factors That Obscure the View
Several external factors related to the fetus and the mother can prevent the sonographer from capturing a clear image. The most common issue is poor fetal positioning, such as the baby having its legs crossed, tightly curled up, or facing away from the ultrasound probe. When the baby’s private area is obscured, the sonographer cannot confirm the sex, often resulting in an inconclusive finding that may require a follow-up scan.
Maternal body habitus can impact the clarity of the ultrasound image, as increased maternal tissue makes it challenging for sound waves to penetrate and return clear pictures. The amount of amniotic fluid surrounding the baby also plays a role. Lower fluid levels reduce the space available for the baby to move, making it difficult to reposition the fetus for a better view of the genitalia. These situational roadblocks increase the likelihood of the scan yielding an inaccurate or uncertain result.
Non-Ultrasound Confirmation Options
For parents who receive an inconclusive result or seek definitive confirmation earlier in the pregnancy, non-ultrasound options are available. The most common alternative is Non-Invasive Prenatal Testing (NIPT), which analyzes small fragments of the baby’s DNA circulating in the mother’s bloodstream. NIPT can be performed as early as the tenth week of pregnancy and determines sex with high accuracy, often reported to be over 99%.
This blood test identifies the presence or absence of the Y chromosome, which genetically determines sex, offering a reliable alternative to visual assessment. More invasive procedures, such as amniocentesis or chorionic villus sampling (CVS), can also determine sex with near 100% accuracy by analyzing fetal cells directly. These methods are primarily diagnostic tools, and while highly accurate for sex determination, they carry a slight risk and are generally not performed solely for this purpose.