Which imaging sign describes a lemon-shaped cranium on fetal imaging?

• A. Lemon sign
• B. Crescent sign
• C. Egg sign
• D. Oval sign

Answer: (A)

A lemon-shaped cranium on fetal imaging is the lemon sign, a ultrasound finding that points to an open neural tube defect such as spina bifida with Arnold-Chiari II malformation. The frontal bones appear flattened and the skull contour becomes curved inward, giving a lemon-like appearance. This shape results from crowding of intracranial contents and reduced cranial vault volume due to downward herniation of the hindbrain. It’s most reliably seen in the mid-second trimester on sagittal views and is often discussed alongside the banana sign, which describes downward displacement of the cerebellum. The other terms describe different cranial contours and are not the same lemon-shaped appearance.