This diagnostic imaging set consists of two grayscale fetal ultrasound images in the transverse plane, focusing on the external genitalia during the second trimester (approximately 18–20 weeks). Image (a) displays the characteristic sonographic appearance of a female fetus. An arrowhead points to a series of parallel, hyperechoic linear structures representing the labia majora and labia minora, often referred to as the 'hamburger sign' in clinical practice. Image (b) displays the sonographic appearance of a male fetus. The arrowhead indicates a distinct dome-shaped structure representing the phallus and scrotum. A central, longitudinal echogenic line is visible at the base of the structure, which is a characteristic finding for male genital development at this gestational age. These images serve as educational examples for fetal sex determination during routine prenatal screening. Key visual indicators highlighted include the presence of parallel lines (female) versus a protruding, dome-shaped morphologic structure (male). The surrounding environment shows typical hypoechoic amniotic fluid and varying echogenicity of fetal soft tissue.

This diagnostic ultrasound illustrates fetal clitoromegaly and labial hypertrophy at 25 weeks of gestation, a key finding in the evaluation of ambiguous genitalia and congenital adrenal hyperplasia. Image (a) provides a sagittal view, highlighting a significantly thickened and prominent clitoris (indicated by a red arrow) protruding from the genital region. Image (b) presents an axial view of the same anatomy, where the enlarged clitoris (red arrow) is flanked by two distinct, hypoechoic, and homogenous labial swellings (white arrows). These findings demonstrate a departure from the normal 'three-line sign' of female genitalia, with the outer labial lines appearing divergent and convex rather than parallel. The imaging modality is B-mode prenatal sonography. This case is educationally significant for distinguishing between female genital virilization and severe male hypospadias during prenatal screening, emphasizing the importance of multi-planar assessment (sagittal and axial) and the identification of internal pelvic organs like the uterus for accurate sex determination.

This diagnostic ultrasound image demonstrates the external genitalia of a female fetus at 30 weeks and 4 days of gestation. The primary sonographic finding is the 'multiple parallel linear echo pattern' (also known as the hamburger sign), which is characteristic of the developing labia majora and labia minora. This pattern consists of three or more hyperechoic, parallel lines representing the labial folds, often separated by a thin hypoechoic midline cleft. The image is captured in a transverse perineal plane, clearly showing the absence of a male phallus or scrotum. This visualization is a key educational marker used in prenatal imaging to confirm female fetal sex and assess normal urogenital development in the third trimester. The surrounding structures include the anechoic amniotic fluid and adjacent fetal limbs, providing context for the perineal orientation.

This composite diagnostic image displays a 20-week gestation level 2 ultrasound scan focusing on fetal perineal anatomy. The image is split into two views: a standard 2D grayscale sonogram on the left and a 3D/surface-rendered reconstruction on the right. In both views, the external male genitalia are clearly visualized and annotated with colored arrows. A blue arrow identifies the penis, which appears as an elongated, echogenic projection. A red arrow identifies the scrotum, seen as a rounded, more inferiorly positioned structure. The 3D view provides enhanced spatial perspective and surface topography of the genital phenotype. This imaging is used during routine second-trimester anatomy scans for sex determination and to screen for genital anomalies. In this clinical context, the visualization of a clear male phenotype is used to confirm fetal sex, particularly when investigating discrepancies with non-invasive prenatal testing (NIPT).

This diagnostic image is a gray-scale prenatal ultrasound from a midtrimester scan, specifically focusing on the fetal perineum to identify external genitalia. The scan displays a transverse view of the pelvic region. Key anatomical features include two parallel hyperechoic (bright) linear structures representing the labia majora. Centrally located between these lines is a smaller, distinct hyperechoic nodule corresponding to the clitoris, forming the characteristic 'hamburger sign' indicative of female fetal sex. The surrounding anechoic (dark) areas represent amniotic fluid, providing the necessary acoustic window for visualization. This image is clinically significant for routine obstetric screening, demonstrating the standard morphological markers used for sex determination and the assessment of external genital development during the second trimester of pregnancy.

This diagnostic ultrasound image shows a midline sagittal view of a human fetus at 13 weeks of gestation during the first trimester. The fetus is oriented horizontally with its caudal end positioned toward the left of the frame. The primary educational focus is fetal sex determination through the assessment of the genital tubercle. A white arrow points to the genital tubercle located at the caudal extremity of the fetal torso. The tubercle is oriented parallel (at an angle of less than 30 degrees) to the horizontal axis of the fetus, which is a sonographic marker indicative of female gender development. The image clearly demonstrates the relationship between the genital tubercle and the lumbosacral spine/rump contour. This technique is clinically significant in prenatal screening for early gender identification and the evaluation of normal external genitalia development.

Diagnostic ultrasound images showing two side-by-side midsagittal views (a and b) of a fetus at approximately 12 weeks of gestation. The fetus is positioned in a supine orientation within the amniotic sac, with visible anatomical landmarks including the head, spine, and caudal region. The images demonstrate specific sonographic markers used for fetal sex determination. Panel (a) illustrates the measurement of the anogenital distance (AGD), shown as a linear caliper measurement (7.40 mm) extending from the anterior edge of the anus to the tip of the genital tubercle. Panel (b) illustrates the genital tubercle angle (GTA), showing an angular measurement (36.47 degrees) formed between a horizontal reference line and the longitudinal axis of the genital tubercle. These sonographic markers are clinically relevant for early prenatal screening and gender identification, as larger AGD and higher GTA values are statistically associated with male fetal development during the first trimester.

This diagnostic ultrasound image shows a midline sagittal view of a human fetus at 13 weeks of gestation during the first trimester. The fetus is oriented horizontally with its caudal end positioned toward the left of the frame. The primary educational focus is fetal sex determination through the assessment of the genital tubercle. A white arrow points to the genital tubercle located at the caudal extremity of the fetal torso. The tubercle is oriented parallel (at an angle of less than 30 degrees) to the horizontal axis of the fetus, which is a sonographic marker indicative of female gender development. The image clearly demonstrates the relationship between the genital tubercle and the lumbosacral spine/rump contour. This technique is clinically significant in prenatal screening for early gender identification and the evaluation of normal external genitalia development.

**Imaging Modality:** Obstetric Ultrasound (2D grayscale). **Anatomical Region:** Intrauterine environment showing a single fetus in a mid-sagittal plane. **Clinical Context:** First-trimester screening, approximately 12–13 weeks gestation. **Key Visual Features:** * **Fetal Anatomy:** The image captures the fetus in a profile view, demonstrating the head (cranium), thorax, and abdomen. The fetal spine is visible along the lower aspect of the trunk. * **Cranial Structures:** Early ossification of the skull is evident. The nasal bone and facial profile are distinguishable. * **Nuchal Region:** The nuchal translucency (NT) space—the sonographic appearance of subcutaneous fluid behind the fetal neck—is visible for assessment. * **Genitourinary Markers:** A prominent genital tubercle (phallus) is visualized at the caudal end of the torso. The cranial orientation of the tubercle (angle relative to the lumbosacral spine) is a sonographic marker used for early fetal sex determination, suggesting male development. * **Environment:** The fetus is surrounded by anechoic amniotic fluid, providing clear visualization of the fetal borders. **Diagnostic Significance:** This plane is critical for assessing crown-rump length (CRL), nuchal translucency thickness, and early anatomical surveys including the presence of the nasal bone and genital tubercle orientation.