**3. Abnormalities of the ventricular system**

*Ventriculomegaly* is the most frequent abnormal CNS finding diagnosed in utero and is the most common indication for second-level neurosonography and fetal magnetic resonance imaging (MRI) [11].

**Figure 12.** Early ventriculomegaly cases.

**Figure 13.** Severe obstructive ventriculomegaly, with unknown origin. Pathologic differentiation of diencephalon and mesencephalon. Conventional necropsy confirms the enlarged ventricles. Microscopy: marked astrocyte cell line proliferation, neuronal migration defects, and cortical fibrosis (stained with hematoxylin and eosin, ob. 40×).

Establishing its class of severity is based on the width of the atrium of the lateral ventricle measurement: ventriculomegaly is considered to be mild when the atrial width is 10–12 mm, moderate between12 and 15 mm, and severe if larger than 15 mm.

In rare cases, ventriculomegaly is accessible in early pregnancy (**Figure 12**).

Yet, this is usually a second- and third-trimester diagnosis. The prevalence of mild ventriculomegaly, based on current criteria, is estimated to be around 0.7% [12]. The finding of ventriculomegaly should trigger a thorough analysis of the fetal brain to investigate all associations (malformative, clastic, tumoral, and syndromic). If no underlying pathophysiology and etiology are found, ventriculomegaly is referred to as "isolated." Melchiorre et al. [13] demonstrated the particularly difficult counseling in such cases. Aqueductal stenosis is the most common cause of ventriculomegaly and its extreme form—fetal hydrocephalus. Published studies of neonates with aqueductal stenosis have noted variable outcomes, with normal development seen in 24–86% of cases [14] (**Figures 13**–**15**).

**Figure 14.** Different cases of unilateral borderline isolate ventriculomegaly, symmetric, and asymmetric.

**Figure 15.** Twin monochorionic diamniotic pregnancy, with discordant major SNC anomaly: severe ventriculomegaly due to aqueductal stenosis. The images display comparatively the same planes: the transthalamic (a) and transcerebellar plane (b), the fetal profile (c) at 17 WA. The transthalamic plane at 25 WA (d). The long-term evolution of fetus B was favorable (after ventriculo-peritoneal shunt).

**Figure 13.** Severe obstructive ventriculomegaly, with unknown origin. Pathologic differentiation of diencephalon and mesencephalon. Conventional necropsy confirms the enlarged ventricles. Microscopy: marked astrocyte cell line

*Ventriculomegaly* is the most frequent abnormal CNS finding diagnosed in utero and is the most common indication for second-level neurosonography and fetal magnetic resonance

**Figure 11.** Imaging the spine in the coronal plane. In left image, a supernumerary lumbar rib case is shown.

proliferation, neuronal migration defects, and cortical fibrosis (stained with hematoxylin and eosin, ob. 40×).

**3. Abnormalities of the ventricular system**

50 Congenital Anomalies - From the Embryo to the Neonate

imaging (MRI) [11].

**Figure 12.** Early ventriculomegaly cases.
