On ultrasound scan performed prior to birth usually before 24 weeks of pregnancy, the babies skull may appear like a lemon. This lemon sign may indicate the presence of spina bifida where the neural tube, the precursor of the central nervous system (brain & spinal cord), has failed to close, usually in the lower back region.
Normally, the fetal skull frontal bones curve outwards (convex), however with spina bifida and other conditions, the frontal bones curve inwards (concave) making the skull look like a lemon in shape.
In the common form of spina bifida – myelomeningocele – patients may have bladder and bowel dysfunction, loss of sensation below the level of the lesion as well as an excess of cerebrospinal fluid in the brain’s ventricles (hydrocephalus).
Reference:Thomas M: The lemon sign. Radiology, 2003 228(1):206-7. Go to reference
Lemon sign A, Spina bifida B