**5. Cortical formation abnormalities**

**Figure 22.** Complex lethal facial and cerebral anomaly. The red arrow indicates the single orbit. The pathologic specimen confirms ciclopy, proboscis, exencephaly (a). Second-trimester case of anencephaly: 2D conventional and 3D ultrasound (b).

**Figure 19.** Rachischisis. Medical termination of pregnancy at 16 WA. 3D ultrasound in a surface-rendering mode (a), pathologic specimen (schisis of the lumbar skin, with exposing the meninges and the spinal canal structures) and MRI

details of specimen, confirming the hemivertebra suspected on US.

54 Congenital Anomalies - From the Embryo to the Neonate

**Figure 20.** Exencephaly. Ultrasound images and the pathologic specimen at 12 WA.

**Figure 21.** Encephalocele at 12 W. Conventional 2D ultrasound and 3D surface rendering.

The cerebral *cortex development* implies evolvement through three steps: neuronal precursor proliferation and differentiation; migration of immature neurons; and cortical maturation (the laminar organization and occurrence of synapsis). Neurons migrate from the ventricular zone (called the germinal matrix) toward the pial surface, along radially oriented glial scaffolds [42–44]. Gyration and sulcation occur afterword, beyond 32 WA. Disruption of any of these steps in cerebral development, due to inherited or acquired causes, can result in a wide spectrum of abnormalities.

*Schizencephaly* is a congenital cerebral defect in clefting, where clefts extend through the hemispheres from the ventricles to the pial surface [45]. Having two clinical types (open and closed), it seems to be caused by a primary failure of development of the cerebral mantle in early pregnancy. The condition is different from *porencephaly*, being characterized by the presence of heterotopic gray matter lining the cleft. Although primary, it has also been reported as a destructive process mediated by vascular injury also.

*Lissencephaly* means literally "smooth brain." This is a rare brain malformation, gene-linked, characterized by the absence of normal convolutions in the cerebral cortex, leading to *microcephaly*. In most cases, neonates have usually a normal sized head at birth. The "cobblestone lissencephaly" is characterized by the irregular surface of the brain on the pathological specimen. This is due to aberrant neuroglial overmigration into the subarachnoid space. The formation of an extracortical agyric neuroglial layer occurs. It seems that the primary cause is the deficit of glycosylation of dystroglycans, resulting in neuroglial overmigration [44–50].

The presence of neurons in any position other than the cortex is called *neuronal heterotopia*. This is caused by an abnormal phenomenon of migration during fetal development. The most frequent type is *periventricular* heterotopia, given by an abnormal development of the neuroependyma [44, 50]. It consists of groups of disorganized neurons and glial cells that are located along the walls of the lateral ventricles. They may be isolated (X-linked and non-X-linked forms) or associated with other CNS malformations. The prevalence in the general population is unknown, but it has been related with epilepsy, seizures, and/ or developmental delay, with different grades of severity. The prenatal diagnosis has been reported, but the condition is underdiagnosed in the vast majority of screening settings. The true *microcephaly* is considered part of a complex disorder [48–51], occurring in syndromes (with or without chromosomal anomalies). It may be associated exclusively with cerebral anomalies (due to either primary cerebral maldevelopment or clastic events like the ischemohemorrhagic ones) or infectious diseases; the latter has gained a particular interest lately, in light of the recent emergence of microcephaly related to Zika virus infection [48–51]. *Macrocephaly* may result from macrocrania, hydrocephalus, or a major subarachnoid space abnormality. If not associated with other conditions, macrocephaly is synonymous with megalencephaly, meaning an increase in the weight and size of the brain [52] (**Figure 23**).

**Figure 23.** Complex cortical anomaly. Nodular periventricular heterotopia seen. Cortical hypoplasia. Microcephaly. Periventricular leukomalacia. The postpartum image highlights the abnormal excessive ossification of the coronal suture and the dysmorphic facial features of the neonate.

**7. Posterior fossa abnormalities**

**8. Vascular abnormalities**

or alloimmune thrombocytopenia [71–79].

Posterior fossa abnormalities include the Dandy-Walker malformation (complete or partial agenesis of the cerebellar vermis, cystic dilatation of the fourth ventricle, and enlarged posterior fossa, with upward displacement of the tentorium, torcula, and transverse sinuses); the mega cisterna magna (a CM measuring more than 10 mm and a normal vermis); the Blake's pouch cyst (the presence of an upwardly displaced normal cerebellar vermis, normal appearance of the fastigium, tentorium, and size of the cisterna magna); and isolated vermian hypoplasia (a normally formed vermis but of smaller size, with an otherwise normal size and anatomy of the posterior fossa) [69, 70]. It seems that the Dandy-Walker malformation, even if apparently isolated on ultrasound imaging, carries a high risk for chromosomal and associated structural anomalies. Isolated mega CM and Blake's pouch cyst have a low risk for aneuploidy and associated structural anomalies. The isolated vermian hypoplasia is extremely rare; thus, the literature does not offer definite conclusions about its significance. This needs to be further assessed (**Figure 26**).

Fetal Central Nervous System Abnormalities http://dx.doi.org/10.5772/intechopen.76208 57

**Figure 25.** Second-trimester alobar holoprosencephaly (a and b). Short and thick corpus callosum (c).

Possible causes of hemorrhage include arteriovenous malformation, benign or malignant, intracranial tumors, fetal infection, drug toxicity, and clotting disorders, such as isoimmune

**Figure 26.** Isolated mega cisterna magna. US and MRI images of the same third-trimester case.

Fetal *hemorrhagic* and *hypoxic-ischemic* insults can lead to antenatal brain damage and fetal stroke. These are associated with fetal death, postnatal seizures, mental retardation, psychomotor delays, and cerebral palsy [72]. Fetal intracranial hemorrhages and strokes can be
