A new sonographic pattern of fetal hydrocephalus-midline-waved sign, and its mechanism were reported in this paper. When the abdominal wall of the pregnant woman was percussed by hand during ultrasound examination, the wave of midline echo was observed on sonoimage. The diagnosis of hydro-cephalus was made by this sign in all 12 fetuses, and confirmed by autopsy. No dilatation of the cerebral ventricle was observed in 5 out of 12 fetuses. It would be difficult to make the diagnosis of hy-drocephalus would previous diagnostic criteria be used. This sign is negative in normal fetuses, therefore-this finding has the practical value in the early and differential diagnoses of hydrocephalus.