What causes the lemon sign appearance on imaging?

• A. Changes in intracranial pressure causing the frontal bones to flatten
• B. Maternal folate deficiency
• C. Fetal skull thickening
• D. Placental insufficiency

Answer: (A)

The lemon sign is an ultrasound finding that reflects altered intracranial pressure and CSF dynamics in a fetus with neural tube defects, most commonly spina bifida with Chiari II malformation. When hindbrain structures herniate downward, the cranial vault experiences changes in CSF volume and pressure, causing the frontal bones to flatten and the skull to take on a lemon-like shape. This anterior flattening is what gives the characteristic appearance on imaging.

This sign specifically points to neural tube defect–related brain and skull changes, not to maternal folate deficiency, placental insufficiency, or fetal skull thickening. Folate deficiency predisposes to neural tube defects but isn’t the direct cause of the lemon-shaped skull; placental issues and skull thickening don’t produce this imaging pattern. The lemon sign often accompanies the banana sign, which reflects hindbrain herniation.