Intrauterine growth restriction (IUGR) refers to growth less than expected for gestational age and is most often defined as EFW under 10th percentile for age. The term IUGR implies underlying pathology, which can be divided into fetal (chromosomal abnormalities, infection), maternal (hypertension, smoking), and placental problems. This is in contrast to the fetus that is small for gestational age (SGA). The latter carries a good prognosis and describes a fetus that is small but has reached its expected genetic growth potential and, after birth, will be well proportioned without other abnormalities.

IUGR may be categorized as symmetric or asymmetric. Symmetric IUGR refers to a fetus that is uniformly small and is typically detected earlier in gestation. Causes of symmetric growth restriction include chromosomal abnormalities, multifactorial congenital malformations, and in utero infection. More common is asymmetric IUGR, which manifests as a small AC in relation to the remainder of the fetus and implies a placental or maternal abnormality. The fetus, receiving less than the required blood supply, preferentially shunts blood to vital organs such as the brain at the expense of the liver, muscle, and fat. If the insult continues, eventually the entire fetus becomes small as it loses the ability to compensate and assumes the appearance of “symmetric” IUGR. This can be differentiated from true symmetric IUGR by history and often by the presence of oligohydramnios, which is a result of chronic stress on the fetus. In contrast, symmetric IUGR is typically accompanied by normal amniotic fluid volume (AFV).

As a single parameter, AC is the most sensitive predictor of IUGR; however, numerous formulas including FL or HC have been used. Slow interval growth may also indicate the presence of IUGR, a helpful measure particularly when gestational age is uncertain.

Close monitoring is recommended in the setting of IUGR. Doppler assessment of the umbilical artery may be used for this purpose and can provide clues to the etiology of IUGR (see Doppler Assessment of Blood Velocities in Umbilical and Fetal Vessels). Other monitoring methods include serial sonographic assessment of growth, nonstress testing, and biophysical profile (BPP) testing. However, antenatal surveillance regimens vary between institutions due to the paucity of randomized controlled trials on this topic (10).

An infant who is macrosomic, or large for dates, is typically defined as one with birth weight >4,000 g or over the 90th percentile for gestational age. Macrosomia is most commonly associated with material diabetes, but other causes include maternal obesity and some genetic syndromes such as Beckwith-Wiedemann syndrome. Fetuses with macrosomia are at risk for shoulder dystocia and brachial plexus injury. As is the case for IUGR, AC is the best single parameter for prediction of macrosomia (11). Weight estimates derived from formulas utilizing biometric parameters in suspected macrosomic fetuses have an error of ±10% (8). Also to be taken into account are factors that affect fetal and birth weight, such as fetal gender, race, and ethnicity. Other methods have been proposed to predict fetal macrosomia, such as measuring subcutaneous fat thickness. The greater availability of 3D-US has also spurred interest in using volumetric measurements for this purpose (12).

With the advent of ultrafast scanning sequences, MRI has become a feasible method of prenatal imaging and a useful adjunct to US. Advantages of MRI include superior soft tissue contrast, larger field of view, and fewer limitations related to the ossifying fetal calvarium, maternal body habitus, fetal lie, and low AFV (22,23).

In determining the appropriate gestational age at which fetal MRI should be performed, consideration should be given to technical factors and the clinical information being sought. Prior to 18 to 20 weeks, detail on fetal MRI can be limited by fetal motion and low resolution due to the small size of the fetus. Although MRI in the third trimester benefits from excellent resolution, a balance should be struck between early diagnosis and waiting past the point of possible fetal intervention.

Although the safety of MRI during pregnancy has not been definitely proven, studies to date have not demonstrated deleterious effects on patients who underwent MRI in utero (24,25). While some animal studies have raised some concerns with regard to potential teratogenic effects, no retrospective human studies have demonstrated adverse findings (26). Another potential safety issue is acoustic damage to the fetus due to the loud knocking noises produced during MRI scanning; however, one small study found no evidence of hearing damage (27). Long-term effects on children who underwent prenatal MRI have also not been demonstrated (27,28,29), although these studies have generally been limited by small sample sizes and short length of follow-up.

After weighing the risks and benefits, fetal MRI can be considered in situations in which it can provide information that is not obtainable by US, that will affect the care of the patient or the fetus, or which will be valuable in counseling families during clinical decision making.

Prenatal screening for abnormalities of the central nervous system (CNS) is typically performed by US on a routine basis or due to elevated maternal alpha-fetoprotein (AFP). Although large CNS anomalies may be visible in the first and early second trimesters, the most favorable time for sonographic evaluation of the CNS is around 20 weeks of gestation (30). Sonographic evaluation of the brain consists of three standard axial sections, with additional coronal and sagittal images as needed. Features of the CNS that can be evaluated by US include brain parenchyma, sulcation pattern, corpus callosum, posterior fossa, and calvarium.

In the presence of abnormal US findings, fetal MRI may provide useful additional information. In the CNS, advantages of MRI include greater soft tissue contrast and the absence of reverberation artifact from the calvarium, making MRI a sensitive tool for evaluating the microstructure and composition of the brain (31). Sulcation pattern is easily seen by fetal MRI and serves as a marker for brain maturity. Ventricular size and configuration are also readily evaluated. Some anatomic structures not routinely depictable by US can be identified on MRI, such as the olfactory bulb, inner ear, optic chiasm, and pituitary gland (22). Signal changes in the brain also mirror the evolving composition of the parenchyma and are due to progressive diminishment in water content, increasing cell density, and myelination (31).

Ventriculomegaly refers to abnormal enlargement of the ventricular system and is a relatively common abnormality detected by obstetrical US. The term hydrocephalus is used specifically in cases due to increased pressure, most commonly due to obstruction to cerebrospinal fluid flow. Ventriculomegaly may also result from abnormal ventricular development or from parenchymal destruction due to processes such as vascular insults or infection. The major causes of ventriculomegaly are aqueductal stenosis, Chiari II malformation, agenesis of the corpus callosum, Dandy-Walker malformation, and fetal aneuploidy (32).

As ventriculomegaly is one of the most common false-positive findings during US fetal anatomic evaluation, accurate sonographic assessment of the ventricular system is key. Measurement of the lateral ventricles is performed at the level of the atrium of the ventricles on an axial view of the fetal head (Fig. 12.5A) (30). Malformations (both CNS and non-CNS) are more commonly found in severe ventriculomegaly (>15 mm) with a rate ranging between 58% and 65% (33). The reported rate of malformations in less severe ventriculomegaly varies widely in the literature but is likely
higher in moderate ventriculomegaly (12.1 to 14.9 mm) than in mild ventriculomegaly (10 to 12 mm) (34). Underlying aneuploidy is also a concern, particularly when additional structural malformations are present.

Fetal MRI is a valuable adjunctive tool in cases of ventriculomegaly to evaluate for etiology and any additional abnormalities (Figs. 12.5B and 12.6). The reported percentage of cases in which MRI detects an additional abnormality not seen on US ranges between 5% and 50% (33).

The most important prognostic indicator is the presence and type of additional structural or chromosomal abnormalities. The outcome of isolated ventriculomegaly, by contrast, varies with the degree of ventricular enlargement. In mild isolated ventriculomegaly, the rates of neonatal and perinatal death and of neurodevelopmental delay are lower when compared to moderate or severe ventriculomegaly (34). A substantial cohort of patients with isolated mild-to-moderate ventriculomegaly also experiences resolution of ventricular dilatation in utero (35).

NTDs result from the failure of full closure of the neutral tube early in gestation. These defects are often first suspected in the face of an abnormal screening maternal serum AFP result. Prenatal US can detect spinal abnormalities, evaluate for associated CNS and non-CNS abnormalities (e.g., clubfoot), and assess fetal growth. Findings of a myelomeningocele include splayed vertebral pedicles and a sac overlying the spinal defect (Fig. 12.7). For a detailed assessment of the spinal lesion, 3D-US is useful because it allows for manipulation of imaging planes after initial acquisition and can be displayed with various rendering algorithms to highlight bony structures or overlying skin. In addition, 3D-US is especially useful in depicting the specific level at which the dysraphic defect lies. Chiari II malformation is commonly seen with an open NTD and comprises downward herniation of the cerebellar tonsils into the upper cervical canal with crowding of the posterior fossa. Intracranial findings include a compressed cisterna magna and an abnormally rounded cerebellum (banana sign), concave contour of the frontal bones (lemon sign), small BPD and HC, and ventriculomegaly (36).

Although some features of Chiari II can be recognized by US, some intracranial findings such as the degree of tonsillar herniation and tectal beaking are better depicted by MRI. Other CNS abnormalities associated with NTD and Chiari II include gray matter heterotopia, callosal dysgenesis, holoprosencephaly, cerebellar dysplasia, and diastematomyelia (37).

Appropriate growth of the lungs requires adequate thoracic space, as well as amniotic and lung fluid to provide positive transpulmonary pressure (38). A variety of fetal chest abnormalities can be detected by US by the second trimester.

The lungs are normally homogeneous and, compared to liver parenchyma, more echogenic on US and hyperintense on MRI. The trachea and main bronchi are depicted as fluid-filled branching tubular structures. The diaphragm can be seen as a thin hypoechoic curved line between the lungs and the abdomen on US. Mediastinal shift and heterogeneity of the lungs can be the first clue to the presence of a space-occupying lesion.