The list of sonographically detectable fetal malformations is truly broad and ever expanding.1 Improvements in resolution and the use of transvaginal ultrasound allow detailed examination of external morphology and internal anatomy, and permit definition of not only major defects, but also more subtle changes that may be harbingers of chromosomal anomalies.2 The importance of early diagnosis lies in the options the knowledge provides: if continuation of pregnancy is contemplated, delivery can be planned in a manner allowing presence of pediatricians and appropriate specialists if deemed necessary, informed decisions regarding monitoring in labor and delivery can be made, therapy can be contemplated in some cases, and termination of the pregnancy is sometimes possible. This article describes selected anomalies based on their relative frequency or their severity in terms of "lethality" or long-term morbidity.
CENTRAL NERVOUS SYSTEM ANOMALIES
The main CNS anomalies are neural tube defects (anencephaly and spina bifida), hydrocephaly, microcephaly, and choroid plexus cysts.
Neural Tube Defects
Neural tube defects are among the most common congenital anomalies in the United States. * Nationally, open neural tube defects occur at an incidence of about 1.2 per 1000 live births, but in some areas, such as the eastern United States, the prevalence can be as high as 1.6 to 1.7 per 1000. In the United Kingdom, this number approaches 2/1000 (down from 4.7/1000 in the late 1960s). Anencephaly and spina bifida account for 95% of the cases of open neural tube defects and encephaloceles constitute almost all the rest. Both anencephaly and spina bifida result from failure of complete closure of the neural tube at about 26 days postconception, Anencephaly is the most common CNS anomaly with a 4:1 female:male preponderance. Diagnosis can be made by ultrasonography as early as 11 to 12 weeks by demonstrating absence of fetal cranium. Associated spinal lesions are present in 50% of cases. Only rarely will an anencephalic infant survive more than a few hours or days.
Figure 1. Spina bifida, cranial signs: longitudinal sonographic section of fetal skull. Note frontal bone scalloping ("lemon sign") indicated by the arrows, as well as the change in theshape of the cerebellar hemispheres ("banana sign") indicated by arrow heads.
Figure 2. Spina bifida: longitudinal sonographic view of the spine. Note the spinal defect and the bulging meningeal membranes (arrows).
Survival and prognosis with encephalocele vary depending on severity (ie, containing fluid or neural tissue) and location. Occipital encephaloceles, the most common location (70% to 75% of the cases), have worse prognosis than frontal ones.
Spinal dysraphism (spina bifida) can occur at any level of the spine. Survival and morbidity depend on the level and size of the defect. The typical shape of the skull (frontal bone scalloping), resulting from decreased frontal pressure in spina bifida, has resulted in the term "lemon sign" while changes in cerebellar morphology give rise to the "banana sign" (Figure 1). Normal cerebellum appears on ultrasound as two adjacent circular structures (cerebellar hemispheres) linked by a "bridge," which represents the vermis. In spina bifida, the bridge as well as the circular structures are pulled posteriorly, resulting in elongation of the hemispheres, which causes the "banana" shape. When these are found, an extremely high degree of suspicion should be entertained for the presence of spina bifida (Figure 2).4
Once an open neural tube defects is diagnosed, a multidiscipiinary team approach is recommended. The team should include a perinatologist, neonatologist, pediatrician, and neurosurgeon, as well as any other necessary support (psychological, social, nurs' ing, etc). Definitive prognosis is difficult to formulate since the presence or absence of limb movements in utero is not a necessary correlate to postnatal findings.5 Prior to viability, the option of termination of pregnancy is available depending on the family's wishes. Mode of delivery is controversial since there are no large studies demonstrating the benefits of abdominal versus vaginal delivery. Delivery at an appropriate center is preferable. The risk of a recurrence after one child with open nueral tube defects is 2% to 4%.
Without therapy, enlargement of the cerebral ventricular system in infants and adults is usually accompanied by a gradual loss of functioning brain tissue and mental deterioration. The natural history in fetuses is less well-defined. Etiology is often difficult to establish even if diagnostic criteria are relatively precise. The two major mechanisms are primary loss of brain substance with excess accumulation of CSF fluid but without elevated pressure and increased pressure secondary to obstruction to flow through and out of the ventricular system with subsequent compression of brain tissue. The latter is the more common. Obstruction can be hereditary (X-linked stenosis of the aqueduct of Sylvius) or a manifestation of intrauterine infection (cytomegalovirus, rubella, syphilis, and toxoplasmosis), chromosomal anomalies, congenital malformations, such as spina bifida (accompanied by hydrocephaly in 80% of cases), Dandy-Walker malformation, and ArnoldOhiari myelodysplasia. The incidence of congenital hydrocephaly is 5 to 25 per 10,000 births. Diagnostic criteria depend on gestational age with enlarged biparietal diameter being a late manifestation. Early diagnosis is based on the measurement of the ventricle width (V) and its ratio to the maximal width of the hemisphere (H). The V/H ratio should be at the most 0.5 between 17 and 20 weeks and 0.33 after 20 weeks.6'7 Mode of therapy will depend on gestational age at diagnosis, severity, and presumed etiology.8 The early hopes that intrauterine shunting procedures would be helpful have not been confirmed, and it is now not a recommended option.9
Congenital Anomalies Associated With Cardiac Defects
Birth incidence is 1 to 2 per 20,000. Prenatal diagnosis is difficult and should be considered if the biparietal diameter falls more than three standard deviations below the mean or if the head circumference demonstrates progressive decreased growth until it falls below the 5th percentile.
Choroid Plexus Cysts
These vary from very small to 1 to 2 cm in diameter. They are often bilateral. The majority resolve spontaneously by 23 to 25 weeks gestation and have no clinical significance. However, if they persist or are associated with other malformation, the risk of fetal aneuploidy according to some authors can be as high as 10%. to The most common karyotypic anomaly is 18.
ANOMALIES OF THE FACE AND NECK
Cleft lip and palate with an incidence of 1 per 800 to 1000 live births are the most common congenital facial malformation. In large studies, more than 60% of infants with cleft lip and palate have other congenital malformations, and trisomies 13 and 18 are common aneuploidies. Prenatal diagnosis is wellestablished, and if made, should lead to an in-depth look at fetal anatomy and consideration for amniocentesis.
Chromosomal Anomalies and the Frequency of Cardiac Defects
The main differential diagnosis of a neck mass includes cystic hygroma, occipital cephalocele, hemangioma, goiter, and teratoma. Cystic hygroma is the most common, it is a benign lymphangioma secondary to congenital blockage of lymphatic drainage. In-utero diagnosis is well-documented. Cystic hygromas are associated with chromosomal anomalies in 80% of cases,11 most commonly monosomy 45XO. Trisomies (in particular 21) and other genetic syndromes often have been described in the presence of cystic hygroma. Prognosis is relatively poor, particularly in the presence of nonimmune hydrops fetalis.
CONGENITAL HEART DISEASE
Congenital heart disease is the most common severe congenital anomaly and occurs in 0.8% of live births. Factors clearly associated with cardiac anomalies are found in 25% to 30%, but the majority occur in patients considered to be at low risk for congenital heart disease. Cardiac anomalies have been reported to be highly associated with other structural anomalies Table 1) as well as with chromosomal abnormalities (Table 2).'2 Some fetuses are at high risk for congenital heart disease and the risk factors may be divided into familial/environmental and fetal risk factors (Table 3).
Prenatal diagnosis of congenital heart disease is important as these malformations are associated with fetal and neonatal morbidity and mortality and their early detection allows for counseling and planning of treatment.
The basic views to be obtained during scanning of the fetal heart are four-chamber view and pulmonary and aortic outflow tracts, including short axis view, parasagittal view, and five-chamber and ductal views.
Four-Chamber View. This is the most important view for screening for congenital heart disease.13 There is controversy regarding the percentages of congenital heart disease that are detected by using routine scanning of a four-chamber view, and these estimates vary from 63% to 96%. The abnormalities most often not detected by the four-chamber view are small septal defects and anomalies involving the great vessels. By obtaining this view, both ventricles, atria, atrioventricular valves (mitral and tricuspid), foramen ovale, and interventricular septum can be visualized. This view also will assist in determining the general orientation of the heart in the chest, its size relative to the chest cavity, and the presence or absence of a pericardial effusion. A small left ventricle is most often a sign of the hypoplastic left heart syndrome, which is a spectrum of anomalies including coarctation of the aorta, and underdevelopment of the aortic, mitral valve, or left ventricle (Figure 3). These latter anomalies often are associated with other fetal anomalies. In hypoplastic left heart syndrome, blood flow is supplied through the ductus arteriosus, and neonatal survival is dependant on a patent ductus. Hypoplasia of the right ventricle is most often due to pulmonary atresia or tricuspid atresia. The severity of these anomalies depends on the presence or absence of a ventricular septal defect.
Interventricular septal defect, Interventricular septal defect is probably the most common structural anomaly of the heart. It can be small or large, isolated, or associated with other cardiac defects. Small defects may not be detected on four-chamber views, but larger defects should be looked for and detected. Color Doppler imaging has been suggested to improve the diagnosis of this defect, but due to the fact that the inutero pressures are similar in both ventricles, flow often is not detected through the defect.
Forman ovaie. The foramen ovale is the opening between the two atria. It allows for bidirectional blood although in the normal state the shunt is predominantly from right to left.
Atrioventricular septal defect. An atrioventricular septal defect can be complete or partial. The partial is relatively benign whereas the complete often is associated with other serious cardiac defects.
Valvular defects. Tricuspid regurgitation is the most common form of atrioventricular regurgitation. The most common etiologies are fetal anemia, ductal constriction, pulmonary hypertension, or congenital heart disease such as Ebstein's malformation (abnormal tricuspid valve), and pulmonary stenosis. Color imaging has been helpful in identifying and visualizing the régurgitant jet and therefore in placement of the Doppler sample volume for flow analysis.
Cardiac tumors. Increased echogenicity within the ventricles often is associated with cardiac tumors. The most common tumor is rhabdomyoma, which is associated with tuberous sclerosis in more than half of the newboms.14 These tumors are usually larger than normal bright echoes often seen in the ventricles, which are thought to represent attachment of papillary muscle. The tumors generally are located closer to the ventricular wall. Other rare cardiac tumors are teratoma, myxoma, and hemangioma.
Risk Factors for Congenital Heart Disease
Cardiomyopathy. Diffuse thickening of the heart muscle, including the septum, is associated with cardiomyopathy. This occurs in diabetic pregnancies, myocarditis from viral and bacterial infections, glycogen storage disease, endocardial fibroelastosis, and obstructive myopathy.
Pulmonary and Aortic Outflow Tracts. Views of the outflow tract confirm that the main pulmonary artery exits the right ventricle perpendicular to the aorta. The normal crossover relationship between the aorta and pulmonary artery is important in rejecting the diagnosis of transposition of great vessels. The major fetal cardiac malformations that may be detected by obtaining outflow tract views are tetralogy of Fallot, truncus arteriosus, transposition of the great vessels, truncus arteriosus, and coarctation of aorta.
ABNORMALITIES OF THE THORAX
Prenatal detection of noncardiac abnormal conditions of the fetal thorax are important as many of them can lead to pulmonary hypoplasia due to lung compression.15 On a normal scan of the fetal thorax, the heart is located mostly in the left thorax and occupies about a third of its volume. Pulmonary hypoplasia is a significant cause of neonatal morbidity and mortality.
Figure 3. Left hypoplastic heart: transverse sonographic view of the fetal chest at the level of the "four-chamber view." Two chambers only are demonstrated (right atrium [ra] and right ventricle [rv]). The left half of the heart is not demonstrated (sp=tetal spine).
The main categories of etiologies that may influence the normal development of the lungs include:
* oligohydramnios secondary to abnormal development of urinary tract or prolonged rupture of the membranes,
* skeletal dysplasias,
* chest masses such as hydrothorax, pulmonary cysts, diaphragmatic hernia, teratomas, and other tumors,
* distention of the abdomen in prune-belly syndrome or abdominal masses, and
* neurologic and chromosomal abnormalities causing decreased breathing.
Thoracic size has been correlated with normal lung development. A small thoracic circumference is sensitive for the diagnosis of pulmonary hypoplasia although it has to be remembered that the size of the thorax can be misleading as far as lung maturity is concerned. The thorax may be enlarged or of normal size in the presence of a tumor in the chest, even though the tumor can compress the lung, which could result in severe lung immaturity.
Bronchopidmonary Foregut Malformation. Pulmonary sequestration is a malformation characterized by the presence of a discrete mass of nonfunctioning lung parenchyma without connection to the normal tracheobronchial tree. The blood is supplied from the systemic arteries and most commonly from the descending aorta below the diaphragm. The difieren' tial dfagnosis includes diaphragmatic hernia and bronchogenic cysts. The two described forms are intralobar and extralobar. In intralobar sequestration, the mass is separated from the bronchial tree but within the normal pleural cavity. In the extralobar type, it is completely separated from the rest of the lung. Most are intrathoracic, although some are reported to be found within or below the diaphragm. Sonographically, this anomaly appears as a homogeneous lung mass, and the final diagnosis is only made after color Doppler imaging reveals the feeding artery.
Other malformations are associated with sequestration; the most common are diaphragmatic hernia, tracheoesophageal atresia, and vertebral and cardiac defects. Sequestrations also may cause mediastinal shifts and have been associated with hydrops. Even though these fetuses may die in utero or shortly after birth due to pulmonary hypoplasia, in up to 65% a decrease in size or disappearance of the mass during pregnancy has been reported.
Bronchogenic cysts are caused by abnormal budding of a segment of the tracheobronchial tree. They may be intrapulmonary or intramediastinal and as such may cause collapse of a portion of the lung due to compression of bronchi.
Cystic Adenomotoid Malformation. This is the most common lung malformation, accounting for about 25% of congenital lung lesions. It is mostly unilateral. The etiology is failure of endodermal bronchiolar epithelium to induce surrounding mesenchyme to form bronchopulmonary segments. The cystic masses in this malformation replace normal pulmonary tissue. Three types have been described:
* Type 1 is single or multiple cysts measuring 2 to 10 cm in diameter. The prognosis after resection is good in these infants if hydrops does not develop.
* Type 2 is smaller cysts of < 1 cm in diameter. The outcome is generally poor.
* Type 3 is microcystic large lesions associated with hydrothorax and pulmonary hypoplasia. It appears echogenic on ultrasonography.
Associated anomalies are mostly seen with Type 2. They are renal agenesis, tetralogy of Fallot, hydrocephalus, congenital diaphragmatic hernia, and skeletal deformities. When bilateral, prognosis is poor due to compression of heart, esophagus, and vena cava, and development of hydrops. Surgical correction is the main treatment option with good outcome. Some have suggested in-utero drainage, but the benefit of this treatment modality is still unknown.
Congenital Diaphragmatic Hernia. Diaphragmatic hernias result from failure of fusion of the diaphragm leaves originating from the pleuroperitoneal canal, which results in communication between the thoracic and abdominal cavities. This occurs around 6 to 10 weeks at the time the intestines return to the abdomen. The most common is the Bochdalek posterolateral hernia occurring in 90% of the cases with 75% to 80% of these on the left side. Most are unilateral, and they occur in 1 in 3000 births.16 Right -sided hernias are less common and, if small, are usually of no immediate consequence. Mediastinal shift is the first abnormality detected on ultrasonography. As most are on the left, they almost always include stomach and small bowel and can be visualized during the ultrasound examination. If the defect is large, the spleen, kidney, and liver may hemiate as well. Congenital diaphragmatic hernia has been associated with other anomalies. The most common are cardiac defects, hydrocephalus, encephalocele, and certain chromosomal abnormalities such as trisomy 13 and 18. The second type is the Morgagni or parastemal hernia, which occurs only in 1 to 100,000 births.
The differential diagnosis of congenital diaphragmatic hernia includes malrotation of the heart and chest masses. If the stomach bubble is not detected in the abdomen and a mass is seen in the thorax, the chances are that it is a case of congenital diaphragmatic hernia. Other uitrasonographic clues are polyhydramnios, small fetal abdomen, and peristalsis in the chest.
The overall mortality of fetuses with congenital diaphragmatic hernia is more than 70%. The primary cause of death is related to pulmonary hypoplasia and persistent fetal circulation due to pulmonary hypertension. Indicators of poor outcome are early diagnosis, hydrops, associated malformations, and dilated stomach in the chest. In-utero corrective surgery has had some success, although the overall benefit of this approach is not clear.
This occurs when the muscle element of the diaphragm is partly or completely missing. The thin diaphragm will extend upward into the thorax with any intra-abdominal pressure. Large eventrations may be difficult to distinguish from congenital diaphragmatic hernia. Eventration can be unilateral or bilateral. The unilateral eventrations are usually asymptomatic whereas bilateral ones have been associated with chromosomal abnormalities, toxoplasmosis, and cytomegalovirus.
Pleural effusion, or fetal hydrothorax, refers to fluid located between the parietal and visceral pleura. Fetal hydrothorax may be primary and isolated, but usually it is secondary and associated with one of the following etiologies: hydrops fetalis, cystic adenomatoid malformation of the lung, congenital diaphragmatic hernia, metabolic abnormalities, hydrocephalus, facial defects, cystic hygroma, placental abnormalities, and chromosomal anomalies such as trisomy 21 and Turner's syndrome. The etiology of hydrothorax often can be diagnosed by detailed uitrasonographic studies, but in many cases a specific cause can not be found. Unilateral and isolated pleural effusion carries the better prognosis and often disappears during pregnancy. Possible etiologies of transient pleural effusion may be undetected infections, episodes of cardiac failure due to arrhythmia, and transient decrease in fetal oncotic pressure. Bilateral effusions with hydrops carry a bad prognosis with overall mortality above 50%. In the newborn, the most frequent cause of isolated effusion is chylothorax, which often leads to respiratory distress.
Differential Diagnosis Between Omphalocele and Gastroschisis
Sonographically, the effusion has a hypoechogenic appearance similar to any other fluid collection. If small, it will follow the contour of the thorax, but when large, it may bulge and change the appearance of the diaphragm and chest wall.
Fetal pleural effusion is associated with pulmonary hypoplasia and poor perinatal outcome. To prevent these complications, some have performed in-utero aspiration to reduce lung compression. The success of this procedure is not uniform, with best results in infants delivered after 32 weeks and who had no hydrops at the time of shunting.
Malformations of the gastrointestinal system comprise two major groups: defects of the abdominal wall17 and obstructions at diverse levels of the intestinal tract.
Ventral Wall Defects
These include omphalocele and gastroschisis. The etiology and significance of these two defects are very different,18The overall incidence is 1 in 3000 to 5000 live births, but omphalocele is twice as common as gastroschisis. Because of possible spillage of alpha fetoprotein from the ventral wall defect into the amniotic cavity, measurement of maternal serum alpha-fetoprotein may be a warning sign to the presence of such a condition. Major differences between the two entities are described in Table 4.
Figure 4. Omphalocele: longitudinal sonographic view of fetal body clearly demonstrating extracorporeal fetal bowels (o=omphalocele) surrounded by a membrane (arrows).
Omphalocele. Omphalocele is essentially an umbilical hernia (Figure 4). It results from lack of closure of the abdominal wall and failure of formation of an umbilical ring at 6 weeks gestational age. It is, therefore, a midline defect with abdominal content herniating into the umbilical cord. This explains the ultrasonographic appearance of bowels OT othei organs covered by a membrane (peritoneum and amnion). The general appearance of the defect remains constant throughout gestation except for normal growth. The major issue to remember with omphalocele is the frequent association with chromosomal aberrations in approximately one third of cases, the most common being trisomies 13 or 18 or other structural anomalies in 50% to 75% cases, particularly neural tube and cardiac defects, as well as gastrointestinal obstructions.
If the omphalocele is isolated, prognosis is favorable for good neonatal outcome. Only if the omphalocele is very big (larger than the fetal head) or if die liver is included should cesarean delivery be considered. Otherwise, there is no documentation that vaginal delivery is detrimental. Immediate assessment of the neonate by a pediatrie surgeon (who, ideally, also was involved in the prenatal course) is essential. The lower two thirds of the neonate should be enclosed in a sterile plastic bag to protect the viscera and prevent massive heat loss. Repair can be performed immediately for small defects or may require decompression and maintenance of the organs extracorporally to allow for staged closure.
CostroscHtsis. Gastroschisis (Figure 5) is thought to be the outcome of a vascular accident in the right umbilical vein at about 7 to 8 weeks gestation. This may be secondary to abnormal return of the bowels to the abdominal cavity from their physiological position in the umbilical stalk. Necrosis of the abdominal wall permits bowels to exit the abdominal cavity, usually to the right of and above the umbilical cord insertion. This explains why the external organs are not covered by a protective membrane as opposed to omphalocele. Free loops of bowel are demonstrated floating in the amniotic cavity. Progressive bowel obstruction may be observed as pregnancy progresses. This may be secondary to irritation of intestinal serosa by amniotic fluid with thickening often visualized. Karyotype anomalies are not associated with gastroschisis, nor other structural abnormalities, except in relatively rare instances. The only frequent association in up to 70% cases is intrauterine growth restriction, possibly due to decreased nutrient absorption.
Cesarean delivery has not been shown to improve outcome and therefore should be performed only for routine obstetrical indications. Postnatal management is initially identical to cases of omphalocele, as above, although electrolytes and fluid balance and heat loss may be more problematic because of the exposed bowels. Surgical repair is, therefore, more urgent than for omphalocele. Here, too, staged reduction may be necessary. Prognosis is excellent, although bowel necrosis and paralytic ileus may make the course of the disease protracted.
The process by which fetal gastrointestinal tract develops is very complex. From the yolk sac being incorporated into the folding embryo at approximately 4 weeks gestation, to the development of a tracheoesophageal septum, recanalization of the physiologically-obstructed duodenal lumen, midgut rotation, and finally, retraction back into the peritoneal cavity at 10 to 12 weeks, any and many steps can go wrong with resulting obstruction at various levels. The major conditions are esophageal atresia with or without tracheoesophageal fistula, duodenal atresia, and small or large bowel obstruction.
Esophageal Atresia. Polyhydramnios and absent stomach bubble are the ultrasonographic hallmarks. Diagnosis will not always be definite because with a tracheoesophageal distal fistula, some amniotic fluid may gain access to the fetal stomach that may be visualized and give a false sense of security. The picture may be part of a more complex sequence such as VACTERL (anomalies of the vertebra[VJ, anorectal area [A]1 heart [C for cardiac], tracheo-esophageal fistula [T], esophageal atresia [E], kidneys [R for renal], limbs [L]) or VATER (vertebral defects [V], anal atresia [A], fistula with esophageal atresia [T-E], radial and renal dysplasia [R]). Immediate stabilization and surgery are necessary.
Duodeno! Atresia. This is associated with trisomy 21 in one third of cases. Polyhydramnios and a double bubble, formed by the dilated stomach cranial to the pylorus and the dilated duodenum, proximal to the atresia are typical ultrasonographic findings.
Figure 5. Gastroschisis: transverse sonographic view of the fetal abdomen. Extracorporeal bowels are not surrounded by a membrane.
Small and Large Bowel Obstructions. These are relatively rare. Small bowel obstruction most often results from a malrotation and will not cause polyhydramntos unless proximal to the ileus. The etiology of large bowel obstruction usually is inspissated meconium or meconium pentamitis more commonly seen with cystic fibrosis. In-utero vascular accident also can result in atresia of an area of the intestinal tract. Immediate repair is necessary after birth, particularly if the etiology is volvulus to prevent more extensive damage to the bowels.
LIMB-BODY WALL DEFECT
This rare anomaly (1 in 14,000), also called body stalk anomaly, is secondary to a failure in the development of the cephalic, lateral body, and caudal folds with extensive abnormality of the anterior abdominal wall. Abdominal viscera are hemiated through the defect and adherent to the amnion and placenta since the umbilicus and umbilical cord are absent. Extensive anomalies of other organs are usually present (central nervous system, heart, limbs, marked scoliosis). Maternal serum alpha-fetoprotein level is elevated, and the condition is universally lethal.
Bladder and cloacal exstrophies can be considered partial forms of the above. In bladder exstrophy (1/30,000 to 40,000), the bladder posterior wall is exposed due to absence of the anterior wall,19 Bladder and lower gastrointestinal tract are involved in cloacal exstrophy, which is rarer (1/200,000), but most of the time is associated with omphalocele (80%), anomalies of the kidneys (60% cases), spine, and cardiovascular system, and therefore has a much worse prognosis. Some cases of extensive postnatal repair have been described with satisfactory results.
Renal abnormalities are relatively common and occur in 7 of 1000 live births. Approximately 50% of uitrasonograprucally diagnosed fetal malformations are renal abnormalities.20 Urine production in the fetus begins around 10 weeks gestation and is the main source of amniotic fluid after 18 weeks.
Fetal Genitourinary Abnormalities and Appearance of the Bladder
The fetal bladder is an easily recognizable structure on ultrasonography, and most genitourinary tract abnormalities will have an impact on the anatomy and function of the bladder. In general, the bladder can be dilated or absent (Table 5).
Genitourinary malformations can be classified as follows:
* renal agenesis or severe hypoplasia,
* abnormal position of normal kidney,
* renal cystic disease (the four Potter dysplasias),
* renal tumors,
* obstructive uropathy,
Renal Agenesis or Severe Hypoplasia
The primary bilateral form of this abnormality occurs with a frequency of 1 in 4000 live births. It also may occur as a part of other congenital syndromes and in fetuses with chromosomal abnormalities. As a consequence of renal agenesis, there will be severe oligohydramnios after 18 weeks, fetal crowding, and the typical Potter syndrome with low-set ears, small or absent bladder, severe limb deformities, and pulmonary hypoplasia. With early onset of oligohydranv nios, the fetal survival rate is low and ranges between O and 15%. The differential diagnosis includes severe growth restriction for chromosomal and other reasons, and ruptured membranes. It should be noted that in the absence of kidneys, the adrenal glands can be mistaken for kidneys, and careful anatomic assessment should be performed in all cases to avoid this mistake.
Unilateral renal agenesis is more common and the diagnosis more difficult as the bladder and amniotic fluid volume appear normal.
Figure 6. Urinary tract obstruction: longitudinal sonographic view of the fetal lower abdomen. Note the enlarged bladder (W) and the hydronephrotic kidney (arrows).
Abnormal Position of Normal Kidney
The most frequent ectopie kidney is a pelvic kidney, and it occurs in 1 in 1000 live births. With the absence of one kidney from its normal location, a common mistake is to make the diagnosis of unilateral renal agenesis before careful scanning for the presence of an ectopie kidney. In the case of a crossed' fused ectopie kidney, it is absent from its normal position and located on the opposite side from its ureter. Fusion of the upper or lower poles of the kidneys results in a horseshoe kidney. This occurs in 1 in 500 live births. Most of these kidneys are asymptomatic and diagnosed only if associated with other anomalies such as cardiac, central nervous system, or chromosomal anomalies.
Renal Cystic Dsease
Renal cystic disease is the second most frequent renal anomaly diagnosed tnutero, following hydronephrosis. There are several different conditions involved, and the ultrasonographic distinction between them is not always possible.
* Potter Type 1. This is called infantile polycystic kidney disease and has an autosomal recessive transmission mode, with a frequency of 1 in 50,000 live births. The normal kidneys are replaced by microscopic cysts that give them the echogenic appearance on ultrasonography- The kidneys are enlarged, and severe oligohydramnios with absent bladder is also found. The outcome of these babies is poor because of poor renal function and pulmonary hypoplasia due to the longstanding oligohydramnios.
* Potter Type U. Also called multicystic-dysplastic kidneys, these kidneys demonstrate multiple large cysts that are noncommunicating with no apparent renal parenchyma. This condition is unilateral in approximately 75% of cases and results from an obstruction early in the first trimester. If oligohydramnios is present, it may be concluded that severe kidney abnormality is also present in the other kidney. If the disease is unilateral with normal amniotic fluid volume, the outcome is good.
* Potter Type /Í. This condition is rarely diagnosed inutero and is also termed adult polycystic kidney disease. This is an autosomal dominant disorder with variable expression. In women with a history of this disease, it has been diagnosed prenatally in some cases and enlarged echogenic kidneys in fetuses at risk should raise the possibility of this condition.
* Potter Type IV. This is renal cystic disease secondary to obstructive uropathy and hydronephrosis in the second and third trimester. Posterior urethra! valves and other obstructive uropathies can cause cystic dysplasia of the kidneys. On ultrasonography, the characteristic sign is the clear communication between the dilated main pelvis and the cystic calyces.
Renal and Adrenal Tumors
The most common tumors are adrenal cysts, mesoblastic nephroma, Wilm's tumor or nephroblastoma, and adrenoblastoma. The ultrasonographic picture of all these tumors is that of a solid mass occupying the renal fossa, and the diagnosis usually is made only postpartum.
Obstructive conditions (Figure 6) of the genitourinary system are commonly diagnosed on prenatal ultrasonography. Depending on the location of the obstruction, its severity, and duration, any or all parts of the urinary tract will appear dilated on ultrasonography.21
Posterior urethral valve is the most common cause for dilated bladder and occurs almost always in male fetuses. If the valve is complete, bladder and ureters will appear dilated, and after some time, the kidneys will appear hydronephrotic. In cases of complete obstruction, oligohydramnios is a prominent feature, whereas in partial obstruction, some amniotic fluid is seen. In the severe cases, urinary drainage is necessary to prevent hypoplastic lungs.
Megacystic-microcolon-intestinal hypoperistalsis syndrome is a fatal disorder with an autosomal recessive inheritance. It is a cause for bladder distention and dilation of small bowel.
Ureterovesical junction obstruction is the most common cause of hydroureter. This condition occurs more frequently in male fetuses and is the result of ureterocele or ureteral stenosis. The characteristic findings are those of hydroureter and hydronephrosis.
Ureteropelvic junction obstruction is the most common cause of obstructive uropathy and hydronephrosis. It occurs more often in males and is unilateral in 70% of the cases. Ureteropelvic junction obstruction is caused by deficiency of muscle fibers at the ureteropelvic junction obstruction as well as local obstruction due to failure of recanalization of the ureter. Minimal pyelectasis (dilation of renal pelvis) may be transient, whereas severe hydronephrosis can have severe consequences. In approximately 20% of the cases, pyelectasis has been associated with other anomalies, as well as with trisomy 2 1 . In most studies, a renal pelvis with a diameter >1 cm is an indication of significant hydronephrosis and possible fetal or neonatal compromise. If the hydronephrosis is unilateral and amniotic fluid volume is normal, intervention usually is not required. With bilateral hydronephrosis, severe oligohydramnios is present, and in-utero intervention versus early delivery are options to be discussed.
The list of abnormalities of fetal extremities is extensive. Some long bone dysplasias are mild while others are very disabling or even lethal. Three main categories exist: anomalies of length, development, and postural deformities.22 Important characteristics to consider when scanning fetal bones are: length, shape, density (mineralization), presence of fractures, or absence of specific bones. The most common congenital dwarfism is achondroplasia, which is autosomal dominant, but is due to a new mutation in 75% to 80% of cases. A general decrease in limb growth should alert to the possibility of this diagnosis. Severe limb reductions can be detected early in gestation by a single measurement: achondrogenesis, asphyxiating thoracodystrophy, chondroectodermal dysplasia, diastrophic dwarfism, camptomelic dysplasia, and thanatophoric dysplasia, which is the most common form of lethal dwarfism. Osteogenesis imperfecta can be one of several related disorders with variable long bone anomalies. In the lethal form, severe bowing and fractures of the long bones are demonstrated, as well as profound hypomineralization (reduced echogenicity). Isolated limb reduction deformities are often part of a genetic syndrome.
This represents 90% of cases of fetal hydrops, especially since the incidence of immune hydrops (Rh isoimmunization) has decreased dramatically in recent years. A multitude of etiologic factors have been described.23 Disease of practically any fetal organ can result in nonimmune hydrops: pleural or pericardia! effusion, ascites, and skin edema.
Karyotyping by any one of several methods (amniocentesis, chorionic villus sampling, or cordocentesis) permits the definite diagnosis of a fetal chromosomal anomaly. Fetai anomalies are known to be associated" with aneuploidy; for instance, cardiovascular malformations in 99% of fetuses with trisomy 18, duodenal atresia in Down syndrome, and cystic hygroma in fetuses with 45,XO karyotype are only a few.
Indications for Targeted Ultrasound
Recently, ultrasonographic signs described as associated with chromosomal anomalies have been reported. In one study, shortened femur (ratio of sonographic measurement over expected length <0.91) and increased thickness of the nuchal fold (>6mm) had an 82% sensitivity and 98% specificity in detecting Down syndrome.2 Therefore, these findings as well as unexplained symmetrical intrauterine growth restriction, cleft lip, cardiac anomaly, diaphragmatic hernia, and omphalocele among others should prompt the examiner to examine the fetus even more closely in an attempt to determine whether anomalies can be grouped into a specific recognizable set. Another finding requiring investigation regarding the possibility of a chromosomal anomaly is the single artery in the umbilical cord.24 In addition to two-dimensional imaging, Doppler velocimetry studies of the umbilical artery blood flow recently have been reported as abnormal in 85% of karyotypically abnormal fetuses.25
AMNIOTIC FLUID VOLUME
Both increased (polyhydramnios) and decreased (oligohydramnios) amounts of amniotic fluid have been associated with fetal congenital anomalies. Anomalies giving rise to polyhydramnios mainly involve defects resulting in swallowing difficulties:
* obstructions in the fetal gastrointestinal system,
* face, head, and neck anomalies, and
* CNS defects.
Multiple other anomalies present polyhydramnios with no clear explanation such as many skeletal or renal disorders.26 Unexplained polyhydramnios in early gestation has been demonstrated in cases of chromosomal anomalies and should alen to the need for further testing.27
The most common etiology of oligohydramnios is rupture of membranes. Other important causes include intrauterine growth restriction, postterm pregnancies, and fetal anomalies resulting in decreased urine production such as renal agenesis, severe renal dysplasia, or urinary tract obstruction. Outcome of pregnancies with oligohydramnios, particularly if diagnosed early, is notably worse than when normal fluid is present.28
Which Patients Should Have Targeted Scans?
The now famous RADIUS study demonstrated that in low-risk pregnancies with good prenatal care, ultrasound will not reduce neonatal mortality; however, even in this population, it demonstrated an increased detection of fetal anomalies.29 Certain patients are at risk of a fetal anomaly and must, therefore, have a specialized scan. Table 6 details the major indications. Ideally, all pregnant patients should have a routine scan for dating and fetal anatomy, but this is the biased opinion of these authors and, at the moment, financial restraints and lack of adequate numbers of qualified sonographers and sonologists may prevent us from attaining this ideal goal.
1. Julian-Reynier C, Marquart-Moulin G, Philip N, et al. Fetal abnormal it íes detected by sonography in low-risk pregnancies: discrepancies between pre- and post-termination findings. Feud Diagn TÍICT. 1994:9:310-320.
2. Benacerraf BR1 Oe] man R, Frlgoletto FD. Soriographic identification of second trimester fetuses with Down syndrome. N Erigí ) Mei 19T7;)17:1371-1376.
3. Main DM, Mennutti MT. Neural tube defects: issues in prenatal diagnosis and counseling. Obstet GynecoL ?986;67:1·16.
4. Nicolaides KH, Gabbe SG, Gutdetti R, Campbell S. Ultrasound screening for spina bifida. Cranial and cerebellar signs. Lancet. !986;i:72-74.
5. Warsof SL, AbramowiczJS, Saycgh SK, Levy DL. Lower limb movements and urologie function in fetuses with neural tube and other central nervous systems defects. Fetal Ther. 1988;3:129-134.
6. Campbell S. Esulf diagnosis oí neural tube detects by ultrasound. Clin Obstet Gynecol, 1977;2ft351-362.
7. Johnson ML, DunneMG. Mack LA, Raschbaum CL Ultrasonic evaluation of fetal ventricular growth. Neuroradiofog}. 1984;2 1:127-131.
8. Abramowici JS, Jaffe R. Diagnosis and intrauterine management of enlargement of the cerebral ventricles. J Fermai Mid. 1988;16:165-173.
9. Seeds JW, Aiizkhan RG. Non-Neural Tube Mei/ormaiioni of the Head, Ntck, and Neural AxU tn Congenital MaJ/ommoíBU; Antenatal Dwgnoiu, Perinatal Management and Counseling. Roclcville, Md:. Aspen Publications: 1990.
10. Perpignano MC. Cohen HL, Klein VR, et al. Fetal choroid plexus cysis: beware the smaller cyst. Redtolegy. 1992;182:715-717.
11. Abramowici JS, Warsof SL, Doyle DL, Smith D, Levy DL Congenital cystic hygn> ma of the neck diagnosed prenatauy: outcome with normal and abnormal karyotype. Prowl Diagn. 1989;9:321-327.
12. Copel JA, PiIu G, Kleinman CS. Congenital heart disease and emacardiac anomalies. Associations and indications for fetal echocardiography. AmJ Obstet GynecaL 1986:154:1121-1132.
13. Copel JA, PiIu G, Green JJ, Hobbins JC, Kleinman CS. Fetal echocardiographiç screening for congenital heart disease: the importance of the four chamber view. AmJObjietGjnecoJ. 1988; 1 57:648-655.
14 McGahanJP. Sonography of the fetal heart: findings on the four chamber view. A7R AmJ Roent. 1991:156:547-553.
15. Clark E, Sabbagha RE, Comstock CH. Abnormalities of the chest. In: Sabbagha RE, ed. Diagnostic Ultrasound Appíiíd to Obstetrics tmd Gynecolog), Third edition. Philadelphia, Pa: JB üppincott Co; 499.
16. Harrison MR. The fetus with a diaphragmatic hernia: pathophysiology. natural history and surgical management. In: Hamson MR, Golbus MS, Filly UA, «k The Unborn Parient: Prenatal Diagtwîïî and Treatment. Philadelphia, Pa: WB Saumfers Co; 1990:295-313.
17. Sermer M, Beniie RJ, Pitson L. Catr M, Skidmore M. Prenatal diagnosis and management of congenita! defects of the anterior abdominal wall. AmJ Obstet Gynecol. 1987:156:308-312.
18. Mann L, Fetguson-Smith MA, Desai M, Gibson AAM, Raine PAM. Prenatal assessment of anterior abdominal wall defects and their prognosis. Prenci Diogn. 1984;4:427-435.
19. Jatte R, Shoenfeld A, Ovadia 1- Sonographic findings in the prenatal diagnosis of bladder exstrophy. Am } Obstet Gynecol. 1990:162:675-678.
20. Yared A1 Barakai AY, Ichikawa 1. Fetal nephrology. In: Eden RD, Boehn FH, eds. Assessment and Core of aie Fetus: Ffijuiolcgicoi, CItnicoJ and MetUcokgal Principies. Norwalk, Conn: Appleton and Lange; 1990:69-91.
21. CortevuV JE, Gray DL1 Crane JP, Congenital hydronephrosis. Correlation of fetal ultrasonography findings with infant outcome. AmJO/BKtGynecoL 1991 i165:384- 388.
22. Jeanty P. Romero R. Fetal limbs: norma! anatomy and congenital malformations. Staun Ultrasound CT MR. 1984;5:253-268.
23. Poeschmann RP, Verheijen RHM, Van Dongen PWJ. Differential diagnosis and causes of nonimmunoiogical hydrops fetalis: a review. Objtef Gynecol Sum, 1991:46:223-231.
24. Sailer DN, Keene CL. Sun CJ, Schwärt! S. The association of tingle umbilical artery with cytogenetically abnormal pregnancies. Am ) Obitet Gynecol. 1990;163:922-925.
25. Abramowici JS, Jaffe R. Dopplet evaluation of fetal anomalies. In: Jaffe R1 Wareof SL, eds. Color Doppler imaging ni Obstetrics and Gynecoiop. New York, N:. McCtaw Hill Ine; 1992:209-242.
26. Hill LM, Bteckle R, Thomas ML, Fires JK. lOlylvydramnios: ul traían icaltv-detected prevalence and neonatal outcome. Obstet Cynecd. 1987;69:21-25.
27. Glanti JC, Abramowicz JS. Sherer DM. The significance of idiopathic midtrimester polyhydramnios. Am ) Permutai. 1994;! 1:305-308.
28. Bastide A, Manning F, Herman C, Lange 1, Morrison I. Ultrasound evaluation of amnlotic fluid: outcome of pregnancies with severe oligohydramnios. Am J Obstet Gynecol. 1986;154i895-900.
29. DeVore GR. The routine anttrraial diagnostic imaging with uknmxmd study: another perspective. Obste! Gynecoí. 1994;84i622-626.
Congenital Anomalies Associated With Cardiac Defects
Chromosomal Anomalies and the Frequency of Cardiac Defects
Risk Factors for Congenital Heart Disease
Differential Diagnosis Between Omphalocele and Gastroschisis
Fetal Genitourinary Abnormalities and Appearance of the Bladder
Indications for Targeted Ultrasound