The standard anatomy scan is a comprehensive ultrasound examination typically performed during the second trimester of pregnancy. Its primary medical purpose is to thoroughly assess the fetus’s growth, development, and overall health, checking for structural abnormalities in the brain, heart, and other organs. While the scan focuses on medical evaluation, determining the fetal sex is a highly anticipated element for many parents. This determination relies on visualizing the external reproductive anatomy once it is distinct enough to identify.
Timing for Sex Determination
Accurate sex determination depends on the completion of external genital differentiation. In early pregnancy, the external genitalia, known as the genital tubercle, appear nearly identical in both male and female fetuses. Differentiation begins around the ninth week of gestation, but visual differences remain subtle for several more weeks. The most reliable window for sonographic sex determination begins around the 16th week of pregnancy. Before this time, the genital structures are very small, and the chance of misidentification is higher. The standard anatomy scan, usually conducted between 18 and 22 weeks, is the optimal time because the external anatomy is fully formed and large enough to be clearly seen.
Specific Visual Cues for a Female Fetus
To identify a female fetus, the sonographer focuses on the positive visualization of the labia and clitoris, a view often colloquially referred to as the “potty shot.” The most definitive sign of a female fetus is the appearance of three distinct white lines in the perineal area on the ultrasound screen. These lines are a cross-sectional image of the external female genitalia.
This characteristic appearance is often described using the non-medical term, the “hamburger sign.” The outer two white lines represent the labia majora, which are the prominent outer folds of skin. The third, shorter line visible between them is the clitoris. Confirmation relies on the presence of these three parallel lines, stacked one above the other, and the simultaneous absence of male organs. The visualization of this distinct three-line structure provides a high degree of confidence in the determination of a female fetus.
Why Results Can Be Inconclusive
A definitive sex determination can sometimes be impossible even during the optimal scanning period due to several external factors that obstruct the view. The most common reason for an inconclusive result is the fetal position within the womb.
Fetal Position and Obstruction
If the fetus has its legs crossed, is facing away from the ultrasound probe, or is curled into an unfavorable lie, the sonographer cannot get a clear image of the perineal area. The clarity of the amniotic fluid and the position of the umbilical cord are also considerations. Additionally, a loop of the umbilical cord may be positioned between the baby’s legs, potentially mimicking male anatomy and leading to a false identification.
Maternal and Technical Factors
Maternal factors can also impact the clarity of the ultrasound images. A higher maternal body mass index requires the ultrasound waves to travel through more tissue, resulting in fuzzier images with lower resolution. Sonographer experience and the quality of the ultrasound equipment are also contributing technical factors that affect the ability to capture a clear, diagnostic image.
Anatomical Ambiguity
In rare instances, temporary anatomical ambiguity can make the determination difficult. Hormones passed from the pregnant person to the fetus can cause the external female genitalia to appear temporarily swollen. This swelling may lead to confusion with male anatomy, particularly on certain three-dimensional scans. If the sex cannot be determined during the standard anatomy scan, a follow-up ultrasound is often recommended.