ABSTRACT
A lemon-shaped skull is a classic sign of an open neural tube defect. The lemon shape is most often seen in the middle third of pregnancy and can resolve in the third trimester. In open spina bifida it is associated with a banana-shaped cerebellum. During routine examination, the size and shape of the skull are assessed, as are the integrity of the skull and mineralization. Normally the skull has an oval shape and is continuously echogenic, with the sutures visible as narrow gaps. Cloverleaf- or trilobate-shaped skull can be associated with thanatophoric dysplasia, a lethal form of skeletal dysplasia. Encephalocele is associated with a defect of the skull. Posterior encephaloceles are much more common than at other sites. The size is variable, ranging from larger than the skull to very small—so as to easily be missed. In some conditions the skull is very poorly mineralized and is easily deformed by the pressure of the ultrasound probe.
