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 early ultrasound marker of open neural tube defects, seen as a lemon-shaped, flattened frontal skull due to the brain’s downward displacement and altered CSF dynamics. Its detectability is time-sensitive because the skull is still relatively pliable and the brain-skull contour is more discerning in mid-gestation. After about 24 weeks, the skull undergoes more ossification and hydrocephalus can evolve, which tends to change or mask this contour. Because of these changes, the sign is most reliable before 24 weeks, so evaluation needs to occur earlier to detect it.