Histological and ultrasound data in pre- and postnatal SBA according to gestational age. Histological and US findings are illustrated in chronological order, according to gestational (GA). The time axis is in the middle part of the figure. Histological data of aqueduct and convexity are indicated at the upper part of the figure. US data of ventricular size (Chiari II malformation, ventriculomegaly and macrocephaly) are indicated at the lower part of the figure. During the first half of gestation, neuroepithelial/ependymal denudation is observed before the onset of Chiari II malformation and hydrocephalus.