The lemon sign refers to the shape of the fetal skull at ultrasonography (US) when the frontal bones lose their normal convex contour and appear flattened or inwardly scalloped.
This gives the skull a shape similar to that of a lemon . The sign is seen on transverse sonograms of the fetal cranium obtained at the level of the ventricles.
The lemon sign has a strong association with spina bifida. Although the exact pathogenesis is unknown, it has been postulated that the decrease in the intraspinal pressure in neonates with spina bifida causes the brain to shift downward. This shift decreases the intracranial pressure, which is reflected onto the fetal cranium. The frontal bones are the most vulnerable to the decreased intracranial pressure and respond by flattening or scalloping inward. As the fetus matures, the lemon sign disappears because the frontal bones become stronger and are able to withstand the decreased pressure.
The lemon sign is not exclusive to spina bifida. It has been seen in a variety of conditions such as encephalocele, Dandy-Walker malformation with encephaloceles, thanatophoric dysplasia, cystic hygroma, diaphragmatic hernia, agenesis of the corpus callosum, fetal hydronephrosis, and umbilical vein varix and two-vessel cord
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