The lemon sign (frontal scalloping) and banana sign (cerebellum wrapped around the brainstem) in the cranial views lead to the spinal lesion; the level of the defect is what counselling needs.
Orient first
- Open (myelomeningocele, myeloschisis) vs closed defects; the cranial signs belong to open ones (Chiari II).
- Ventriculomegaly and small biparietal diameter are common.
- Anencephaly and encephalocele are the other neural tube defects seen at the anomaly scan.
Acquire the study
- Transabdominal anomaly scan: transventricular and transcerebellar axial views of the head; sagittal, coronal and axial views of the whole spine.
The manoeuvre
- Transventricular view: frontal bone contour (lemon sign) and atrial width in mm.
- Transcerebellar view: cerebellum shape (banana sign) and the cisterna magna depth in mm (obliterated).
- Spine in three planes: splaying of the posterior ossification centres, a cystic sac, loss of skin covering.
- Level of the defect: count vertebrae from the last rib-bearing (T12) or from the sacrum.
- Feet (talipes) and lower-limb movement.
What confirms it
- A spinal defect with posterior element splaying and the cranial signs.
What licenses you to exclude it
- A normal cisterna magna and cerebellum on the transcerebellar view make open spina bifida very unlikely.
The classic misread
- Missing a small defect when the fetus lies spine-down — rescan.
- Forgetting that the lemon sign disappears later in pregnancy.