Frontal bone convacity due to inward scalloping of the frontal bones of the calvarium such that the head appears shaped like a lemon. It may be visualized when the spinal defect is small and difficult to see.



Concave Contour

Flattened Contour

Temporal area widest part of cranium.
Almost always pathological.
Seen earlier in the second trimester.

Cranium sides more flattened.
Seen at a later stage.
May be seen in pathological or normal pregnancy.



True lemon sign

Lemon Skull- Normal variant

  • Normal cisterna magna.
  • Normal cerebellum.
  • No ventriculomegaly.





False Positive Open Spinal Defect:

  • Lemon sign (normal variant).
  • Appearance of hanging choroid plexus due to non-visualization of the medial wall of the lateral ventricle.
  • Normal cerebellum and cisterna








1. ? arise by same mechanism that produces the Arnold Chiari II (ACII) malformation.

Spinal cord tethered by meningomyelocele

Growth and elongation of fetal spine

Downward displacement of caudal intracranial structure

Decreased pressure in subarachnoid space

Decreased pressure in calvarium

Indrawing of pliable frontal bones LEMON SIGN

As the gestation progresses: Progressive hydrocephalus and increased intracranial pressure combined with thickening of calvarium results in resolution of the sign in late 2nd or 3rd trimester.

2. Due to a mesenchymal dysplasia of cranium (pressure of the classical craniolacunia) i.e. primary skeletal developmental dysplasia.

Initially thought to be highly specific for Arnold Chiari II malformation, as it is not seen in any of the form of hydrocephalus, however:

  1. Seen in the 1-2% N fetuses.
  2. Seen in cephalocele (33%).
  3. Seen in diffuse anomalies.