Why does the lemon sign need to be evaluated before 24 weeks gestation?

Why does the lemon sign need to be evaluated before 24 weeks gestation?

• A. After 24 weeks the skull becomes stronger and hydrocephalus develops, pushing on the flattened skull
• B. AFP testing becomes unreliable after 24 weeks
• C. The lemon sign disappears after 24 weeks
• D. Ultrasound quality is poor after 24 weeks

Answer: (A)

The lemon sign is an ultrasound marker of open spina bifida tied to the shape of the fetal skull. It reflects frontal skull flattening caused by the posterior fossa crowding and the brain changes associated with Chiari II malformation. This skull contour is easiest to recognize when the skull bones are still relatively pliable in mid-gestation. As pregnancy advances, the calvarium ossifies and intracranial dynamics change with potential progression of hydrocephalus, which can alter or mask the characteristic skull shape. Because of this, the sign is most reliably seen earlier, before 24 weeks, so detecting it then provides the best chance to identify the anomaly.