A developing fetal brain is rapidly changing during pregnancy. However, it is very challenging to monitor this important and rapidly changing fetal organ by conventional transabdominal ultrasonography. In this respect, transvaginal high-resolution ultrasound (US) and three-dimensional (3D) US turned out to be very helpful in the assessment of 3D configuration of fetal brain. 3D US is effective in the prenatal assessment of fetal brain anomalies, vascular malformations, and acquired insults [26].

US examination has been widely used in the detection of brain lesions in fetuses and newborns. According to a prospective study that examined the frequency of echodensities in the periventricular white matter, basal ganglia, and thalamus of 124 fetuses with risk of preterm birth, echodensities were found in 66% of the cases by transvaginal US examination. The gestational age window of the fetuses was between 26 and 34 weeks, and all mothers were with either hypertensive disorders of pregnancy or preterm labor. The most frequent region was the periventricular area (52%). At least 50% of the periventricular echodensities persisted after delivery. The study showed that echodensities in several areas of the brain are a relatively common finding in high-risk fetuses [27].

Padilla-Gomes et al. [28] compared the frequency of transient periventricular echodensities (TPE), PVL, and hemorrhagic brain lesions in FGR preterm babies and in appropriate-forgestational-age (AGA) babies. The gestational age of the study population ranged between 24 and 34 weeks, and brain changes were assessed by US at day 3 (US-I), 2 weeks (US-II) after delivery, and term-equivalent age (US-III). They found a higher prevalence of TPE at US-I and PVL at US-II and US-III in FGR neonates. Based on the results, the investigators proposed that fetal growth restriction is associated with an increased prevalence of white matter damage (WMD) in US brain scans of preterm babies [28].

A series of EPIPAGE cohort of French studies confirmed brain abnormalities in very preterm infants according to clinical parameters such as gestational age, plurality, and fetal growth restriction. Based on cranial US findings, the frequencies of WMD, major WMD, cystic PVL, periventricular hemorrhage, and intraventricular hemorrhage were 21%, 8%, 5%, 3%, and 3%, respectively. The risk of WMD showed an inverse relationship with gestational age, while the incidence of cystic PVL in FGR babies and in AGA babies did not differ [29]. Based on the neonatal US findings, 17% of children with grade III intraventricular hemorrhage and a quarter of children with WMD developed cerebral palsy. In contrast, CP was found in only 4% of children with unremarkable US findings [30].

Certain placental histopathological findings are associated with fetal and perinatal brain injuries, and brain US findings were shown to be associated with placental histopathological changes consistent with placental underperfusion or inflammation. A prospective analysis of fetuses (n=77; gestational age, 26 to 34 weeks) demonstrated that moderate brain echogenicity changes such as periventricular echodensity grade IB and intraventricular echodensity grade II and III are found in cases with uteroplacental underperfusion and inflammation. In particular, placental lesions were present in all cases with grade IB periventricular echodensity [31].

The whole fetal brain structure can also be observed by magnetic resonance imaging (MRI) in the second half of pregnancy [26]. Acute chorioamnionitis is a histological surrogate of intrauterine infection and inflammation and is a component of fetal inflammatory response. This is more commonly found in preterm birth. Anblagan et al. [32] examined placental histology and neonatal brain MRI data in a cohort of preterm babies (n=90) using tract-based spatial statistics to compare fractional anisotropy (FA) data and computational morphometry analysis. The volumes of whole brain, tissue compartments, and cerebrospinal fluid were assessed to determine if acute chorioamnionitis is a risk factor for preterm brain injury. The study decreased FA in the genu, cingulum cingulate gyri, centrum semiovale, inferior longitudinal fasciculi, limbs of the internal capsule, external capsule, and cerebellum in cases with acute chorioamnionitis (p<.05, corrected). This suggests that prenatal white matter injury occurs in a substantial proportion of preterm infants [32].

Banovic et al. [33] analyzed the incidence of fetal brain injury by MRI in the cases of preterm labor, preterm prelabor rupture of the membranes, and FGR and compared fetal brain MRI with other antenatal monitoring tools such as US and biophysical profile. They looked at both immediate neonatal outcome and long-term neurodevelopmental handicap at 24 months in 70 cases. While no correlation was found between abnormal MRI and other surveillance tools or immediate neonatal outcome, binary logistic regression showed that fetal brain MRI is the most powerful indicator of long-term neurodevelopmental handicap [33]. A review of prenatal MRI data by Doneda et al. [34] also suggested that transient venous hypertension in utero is responsible for frontal PVL. The investigators looked for anomalies distributed in the deep medullary vein territory in 78 fetuses with unequivocal cerebral clastic lesions [34].

The major reasons for the predominant involvement of cerebral white matter in preterm babies are considered to be the vulner-ability of premyelinating oligodendrocytes to reactive oxygen species and cerebral anatomical constitution. Preterm newborns have impaired autoregulation of cerebral blood flow, and there are vascular end zones and border zones [35,36]. Regarding the etiology of white matter injury of prematurity, accumulated evidence strongly indicates that there are two main pathways involved. One is the ischemic pathway, the other is the inflammatory pathway, and these two pathways can be synergistic [37-39]. The incidence of hypoxic ischemic brain injuries is higher in fetuses exposed to maternal inflammation and infection [40,41].

The ischemic pathway is mainly related to excitotoxicity and oxidative stress [42-44]. Excitotoxicity is due to depolarization following energy failure of the cells. Depolarization leads to excessive glutamate discharge and calcium influx into the cells, as a result of which nitric oxide synthase is activated. Cellular injury is then induced by increased nitric oxide production. Reperfusion injury following H-I cycles imposes oxidative stress on the cells [45]. Premyelinating oligodendrocytes are more susceptible to oxidative stress than fully differentiated oligodendrocytes are because they lack antioxidant enzymes such as superoxide dismutase and glutathione peroxidase [38,46].

A substantial role for inflammatory pathway related to infection and inflammation has been described. Several studies have shown that intra-amniotic infection and inflammation are causative factors of perinatal and long-term complications [47-49].

While it is generally considered that perinatal brain injury is a consequence of intrapartum or postpartum events, there is substantial evidence that a certain proportion of brain injuries has antenatal components [32,50]. An examination of 58 stillborn fetuses for the presence of GMH, pontosubicular necrosis (PSN), and PVL revealed at least one lesion in 40% of cases, clearly indicating that GMH and PVL can occur in utero. The investigators have also found evidence for the prenatal occurrence of GMH in some cases by US examination and pointed out that PVL and PSN can occur in utero as well [51]. Nakamura et al. [52] have confirmed cystic brain lesions in two autopsy cases. One was a donor fetus in twin-to-twin transfusion syndrome, and the other was a case of thanatophoric dwarfism. Chronic PVL was found in the second case. The findings observed in these cases imply that cerebral circulatory disturbance is a pathogenetic mechanism of in utero brain injuries [52].

Several studies have documented the importance of placental pathology in white matter injury of prematurity. Chang et al. [53] looked at neuropathological findings in 37 third-trimester fetal deaths in conjunction with their placental lesions. There was a correlation between neuronal karyorrhexis or white matter gliosis and severe placental inflammation, and histologically proven PVL was found in two cases [53]. An analysis of 167 preterm babies born between 23 and 34 weeks of gestation revealed a significant association between PVL and chronic deciduitis [54]. In preterm infants, antepartum bleeding of placenta previa is a risk factor for PVL [55]. When Wharton et al. [56] performed a case-control study to examine the relationship between PVL and chorioamnionitis in very low-birth-weight infants, severe umbilical cord inflammation was found to be a risk factor of PVL. Kumazaki et al. [57] reported massive retroplacental hemorrhage, extensive infarction, and severe perivillous fibrin deposition in preterm infants with ante- or peripartum PVL. The study findings suggest an association between poor placental perfusion and white matter injury of prematurity.

Experimental studies have been done to determine if in utero brain injuries are induced in animals. Regarding the consequences of fetal brain injury following in utero hypoxia and ischemia, studies have documented both pathological lesions of the brain and the derangement of motor function in rabbits akin to human CP [58,59]. Brain lesions similar to human white matter injury have been easier to reproduce in gyrencephalic animals such as rabbits, dogs, and sheep than in rodents. White matter injuries in models of H-I pathway by hypoperfusion are characterized by more diffuse microglial response, while the experimental simulation of inflammatory pathway using lipopolysaccharide induces lesions with more prominent inflammatory cell infiltration [60]. Rabbits have more traits that can be helpful in studies than other animal species do. Principally, their motor development occurs in the perinatal period, as is the case with humans [61].

Using a model of placental insufficiency, Buser et al. [62] demonstrated selective patterns of gray and white matter injury after global H-I in fetal rabbits. Gray matter injury predominated following H-I at embryonic day 22 (E22), while white matter injury was minimal. They also observed that, following H-I at E25, there was an increased acute white matter injury instead. Consequently, white matter atrophy was detected at E29 in preterm rabbits after H-I at E25, while it was not detected following H-I at E22. As oligodendrocyte progenitors density increases between E24 and E25 in rabbit forebrain, the investigators proposed that it explains the differences in susceptibility to gray and white matter injury and that this may be the case in white matter injury of preterm newborns [62].

Derrick et al. [63] induced motor deficits in rabbit fetuses using a model of in utero placental insufficiency that were akin to motor deficits in human CP in premature and term babies. Surviving preterm rabbit fetuses (67%–70% gestation) subjected to persistent global hypoxia had hypertonia and abnormal motor control. At postnatal day 1, the pups of hypoxic groups had impaired locomotion, motor reflex, sucking, and swallowing. Histological examination revealed acute injury to motor pathways in the subcortical region [63]. In a following study, Derrick et al. [61] modeled sustained and repetitive in utero H-I resembling placental abruption and labor in the pregnant rabbit. They showed that sustained H-I at E22 and at E25 induces fetal death and other deficits in the surviving animals. Magnetic resonance imaging (MRI) suggested that injury in the internal capsule white matter is responsible for a part of the hypertonia [61].

The role of intrauterine infection in fetal brain injuries also has been examined in several animal models [64-66]. Field et al. [67] introduced Gardnerella vaginalis into the pregnant rabbit uterus at E20 or E21 by hysteroscopy to see the effects of intrauterine infection on feto-maternal outcome. Both amnionitis and deciduitis were induced by G. vaginalis inoculation, but maternal fever and preterm delivery surprisingly were not. In the fetuses, however, intrauterine infection with G. vaginalis decreased the live birth rate, and the fetuses exposed to deciduitis had lower birth weight. In addition, the G. vaginalis–inoculated study group had significantly higher frequency of serious brain injury than the control group (60% vs 0%). The study findings indicate that G. vaginalis has more pathological impact in the rabbit fetus than in the mother [67]. There is further experimental evidence that intrauterine infection leads to WMD in utero. Yoon et al. [68] introduced Escherichia coli into the pregnant rabbit uterus from E20 to E21, maintained the pregnancy for additional 5 to 6 days by antibiotic treatment, and examined the brains of fetuses and the placentas. Histologically proven white matter pathology was found in 12 fetuses of 10 E. coli–inoculated rabbits, but not in the control group (p<.05). All of the rabbits with white matter pathology were associated with intrauterine infection [68]. Intrauterine endotoxin administration also induced microstructural changes in the white matter of rabbit newborns, which were detected by diffusion tensor MRI. Term newborn rabbits prenatally exposed to endotoxin at E28 showed decreased FA in periventricular white matter. Brain sections disclosed more frequent activated microglial cells, which may explain the change in diffusivity [69].