**2. Systemic review of congenital anomalies on ultrasound**

#### **2.1 Central nervous system (CNS)**

The frequency of central nervous system anomalies varies according to geographic area and race. It is approximately 1–2:1000 newborns. Survivors are often severely disabled, necessitating long-term care.

The fetal brain undergoes major developmental changes during pregnancy. At 7 weeks of gestation, a sonolucent region is demonstrable in the cranial pole representing the fluid-filled rhombencephalic vesicle. At 9 weeks, demonstration of the convoluted pattern of the three primary cerebral vesicles is possible. The most prominent structures from 11 weeks are the echogenic choroid plexuses which fill the lateral ventricles. In the early second trimester, the lateral ventricles and choroid plexuses decrease in size relative to the brain mass.

Effective ultrasound screening for CNS anomalies can be carried out by examination of two important axial planes through the fetal brain; the transventricular and transcerebellar planes. Therefore, familiarity with the normal appearance of the fetal brain in these planes and at different gestational ages is vital for prompt identification of congenital anomalies.

The transventricular plane is at the level of the ventricular atria, with the echogenic choroid plexus being the dominant landmark. Measurements of atrial diameter made perpendicular to the walls should not normally exceed 10 mm.

This plane is obtained by axial sonogram at the level of the cavum septum pellucidum and shows the lateral margins of the frontal horns, the medial and lateral limits of the posterior horns of the lateral ventricles, and the choroid plexuses. It is used for fetal biometrics and quantification of the ventricular width (**Figure 1**).

The second crucial axial plane is the transcerebellar plane, which allows the examination of the midbrain and posterior fossa. The anatomic landmarks are the inferior portion of the third ventricle and the cerebellar hemispheres, which are outlined by fluid in the cisterna magna. The normal cisterna magna measures 2 to 11 mm in width (**Figure 2**).

A small cisterna magna (<2 mm) suggests a Chiari II malformation. However, it may also occur in massive ventriculomegaly. A large cisterna magna (>11 mm) may be a normal variant (megacisterna magna) or indicate a variety of anomalies. These

#### **Figure 1.**

*Obstetric ultrasound showing biometric measurements.* Fetal head: *Sonograms of the fetal head at the level of the thalami (T) showing measurement of the head circumference.*

*Antenatal Diagnosis of Congenital Anomalies on Ultrasound Screening DOI: http://dx.doi.org/10.5772/intechopen.98586*

#### **Figure 2.**

*Obstetric ultrasound; trans-cerebellar plane: This shows the cerebellar hemispheres (CB), cisterna magna (CM, between the two calibers) and thalami (T).*

include Dandy-Walker malformation, arachnoid cyst, and cerebellar hypoplasia. When these two planes are anatomically normal, the risk of CNS anomaly is minute (0.005%).

Additional scanning planes along different orientations may be required occasionally, when one needs better definition of intracranial anatomy detail.

A sagittal and/or coronal view of the entire fetal spine should be obtained. In the sagittal plane, the normal spine has a 'double railway' appearance and one can appreciate intact soft tissue above it. In the coronal plane, the three ossification centers of the vertebra are visualized and should tether down into the sacrum. These views are used for assessment of vertebral integrity which rules out spina bifida. The presence and regularity of the whole spine from these views rules out sacral agenesis and scoliosis.

#### **2.2 Cardiovascular system**

The heart is the first functional organ in the embryo. It is prominently located and has visible contractions, which has made it the object of study for centuries. However, as recently as the late 1970s and early 1980s, the field of heart development was still in its infancy. In 1984, Dr. Constance Weinstein and colleagues at the National Institutes of Health (NIH) organized a meeting to bring together both cardiologists and basic scientists to summarize what was known about the development of the heart.

Congenital Heart Disease (CHD) is the most common birth defect comprising over 30 types of defects, ranging from mild to severe. It is found in 5–10 per 1,000 live births and in about 30 per 1,000 still births. Some forms of CHD resolve spontaneously, others are fully correctible, while others are life-threatening. Nearly 3 children out of every thousand live births have severe CHD requiring early surgery, while another 13/1000 requires surgery or catheter-based intervention later in childhood.

Precise US diagnosis of fetal heart abnormalities often requires specialized equipment and a high level of expertise.

The examination of the fetal heart begins with the assessment of the disposition of abdominal and thoracic organs. Abnormal disposition is associated with complex

#### **Figure 3.**

*Obstetric ultrasound; 4-chamber heart. Normal four-chamber view of the heart. Transverse scan through the fetal heart showing right ventricle (RV) anteriorly, left ventricle (LV) on the left, right atrium (RA) on the right and the left atrium (LA) as the most posterior. The spine (SP) and left lung (LL) are demonstrated.*

cardiac anomalies. A transverse scan of the upper abdomen allows demonstration of the situation of the liver, stomach and great abdominal vessels. An axial section of the chest demonstration of a four-chamber view of the fetal heart. The heart covers about one third of the thorax. It is shifted to the left part of the thorax with the apex directed to the left.

About 90% of ultrasonographically detectable fetal cardiac defects demonstrate some abnormalities in the four-chamber view. Heart rate and rhythm are assessed subjectively. M-mode is useful for the evaluation of abnormal cases though is of little help in assessing morphology of the heart (**Figure 3**).

#### **2.3 Pulmonary abnormalities**

The lungs are interrogated in the same section used for the four-chamber view of the heart. The fetal lungs are uniformly echogenic. At 18–23 weeks, the middle third of the thoracic area at the level of the four-chamber view is occupied by the heart and the remaining two thirds by the lungs.

#### **2.4 Anterior abdominal wall**

The prevalence of anterior abdominal wall defects in sub Saharan Africa is not known as there are no population based studies. This could be due to nonavailability of prenatal diagnosis and poor outcome of care post-delivery [24]. In a study carried out in a Nigerian tertiary hospital, omphalocele was the most common anterior abdominal wall defect seen [25].

Normal development of the anterior abdominal wall depends on the fusion of four folds (cephalic, caudal and two lateral). Developmental midgut herniation occurs at 8–10 weeks with subsequent retraction into the abdominal cavity at 10–12 weeks.

The integrity of the abdominal wall should always be adequately demonstrated. This is achieved via transverse scans which demonstrate the insertion of the umbilical cord. The urinary bladder should be visualized within the pelvis which rules out bladder exstrophy.

A study conducted to determine the ability of routine obstetric ultrasound to detect and accurately describe fetuses with anterior abdominal wall defects

#### *Antenatal Diagnosis of Congenital Anomalies on Ultrasound Screening DOI: http://dx.doi.org/10.5772/intechopen.98586*

demonstrated examination between 16 and 22 weeks gestation detected 60% of defects with a false positive rate of 5.3%. Fetuses with gastroschisis were incorrectly assigned as exomphalos in 14.7% of cases recognized before 22 weeks gestation. The diagnosis was completely accurate in 71.6% of cases. Therefore, problems of diagnostic accuracy should be put into consideration when counseling couples with a fetal anomaly.
