Which pair of signs on fetal ultrasound are classically described in open neural tube defects, with the cerebellum appearing banana-shaped and the frontal bones forming a lemon sign?

• A. Banana sign and lemon sign
• B. Snowman sign and target sign
• C. Cloverleaf sign and mickey sign
• D. Double bubble sign and target sign

Answer: (A)

Open neural tube defects disrupt the normal flow and containment of brain tissue, leading to characteristic skull and hindbrain changes on fetal ultrasound. The cerebellum becomes crowding and downwardly displaced, producing a curved, banana-shaped appearance—this is the banana sign. At the same time, the frontal bones are pressed inward, giving the skull a lemon-like contour, known as the lemon sign. Together, these signs are classic for open neural tube defects such as myelomeningocele and are frequently seen with associated hindbrain herniation (Arnold-Chiari II malformation).

Other sign pairs describe findings in different conditions (for example, signs linked to duodenal atresia or other fetal anomalies), so they don’t fit this particular pattern.