Renal agenesis, which may be unilateral or bilateral, results from failure of the ureteric bud to develop during early embryogenesis. Unilateral agenesis occurs in approximately 3 per 10,000 births. When this abnormality is present, the single kidney hypertrophies in utero and renal function is normal. There are often associated genital tract anomalies, including bicornuate uterus or other uterine duplication anomalies. Prognosis for unilateral renal agenesis is excellent.

Bilateral renal agenesis is a lethal anomaly characterized by failure of development of both kidneys. This anomaly occurs more frequently in males than females, with a ratio of approximately 2:1. Bilateral renal agenesis leads to severe oligohydramnios, leaving no buffer of amniotic fluid between the uterine wall and the fetus. The resulting pressure on the developing fetus causes a number of fetal deformities, including pulmonary hypoplasia, abnormal facies, and limb positional abnormalities, such as clubfeet. The combination of bilateral renal agenesis and these related anomalies has been termed Potter syndrome. The fetus with bilateral renal agenesis can survive until birth, and then generally dies shortly thereafter from pulmonary hypoplasia. Recurrence in subsequent pregnancies is rare.

Sonographic diagnosis of unilateral renal agenesis is made on the basis of nonvisualization of one of the fetal kidneys (Figures 14.1.1 and 14.1.2). Color Doppler can provide supporting
evidence for this diagnosis by identifying a renal artery on only one side of the aorta (Figure 14.1.2C). The solitary kidney is typically compensatorily large for gestational age.

Two potential errors must be avoided in diagnosing unilateral renal agenesis: missing the diagnosis by mistaking an adrenal gland for a kidney and erroneously diagnosing unilateral agenesis when one of the kidneys is in an ectopic location (e.g., pelvic kidney). The first error can result from the fact that the adrenal gland tends to assume a flattened (“lying-down”) configuration in the renal fossa (Figure 14.1.2D). Mistaking the flattened adrenal for a kidney can be avoided by recognizing that the adrenal gland does not have the internal architecture of a kidney, including renal cortex, pyramids, and central sinus. The second error can be avoided by scanning the fetal pelvis and lower abdomen to be sure that the missing kidney is truly absent rather than ectopically located.

An entity related to unilateral renal agenesis is hypoplasia of one of the kidneys. In such cases, one of the kidneys is normal in size and the other kidney is small (Figure 14.1.3).

Bilateral renal agenesis can be diagnosed by ultrasound from approximately 16 weeks gestation onward. The diagnosis is established by nonvisualization of the kidneys and
urinary bladder in conjunction with severe oligohydramnios (Figure 14.1.4). Other sonographic findings often seen with bilateral renal agenesis include a narrow, elongated head (dolichocephaly) and a small thorax, due to uterine compression on the fetus. Because bilateral renal agenesis is a fatal anomaly and the fetus is suboptimally imaged when it is not surrounded by amniotic fluid, the diagnosis of bilateral renal agenesis should be made only after a careful search of the fetal abdomen and pelvis for the kidneys and bladder. As with unilateral renal agenesis, it is important to avoid mistaking a “lying-down” adrenal gland for a kidney on one or both sides of the abdomen.

Dilation of the fetal urinary tract refers to distension of the intrarenal collecting system, ureters, and/or bladder. The term “hydronephrosis” has been used to refer to dilation of the urinary collecting system, but this term is nonspecific, as it is sometimes used to refer to any urinary tract dilation and other times to refer specifically to urinary tract obstruction, and, therefore, the term may be confusing. Urinary tract dilation can result from urinary tract obstruction, vesicoureteral reflux, or deficient musculature in the walls of the urinary tract and abdomen (prune belly syndrome). When urinary tract dilation results from obstruction, the most common site is at the ureteropelvic junction (UPJ). Obstruction may also occur within the ureter, at the ureterovesical junction, or in the urethra. When the urinary dilation results from vesicoureteral reflux, the dilation may involve the ureters only, or both the ureters and kidneys.

When evaluating urinary tract dilation, the anteroposterior (AP) dimension of the fetal renal pelvis should be measured, because this is an important predictor of postnatal outcome. In addition, the central and peripheral calyces should be assessed for dilation, as well as the ureters and bladder. The renal parenchyma should be examined for abnormalities, such as cysts or increased echogenicity. Based on the AP dimension of the renal pelvis and other sonographic findings, urinary tract dilation can be graded to predict the risk of postnatal uropathy as follows:

Because urinary tract dilation may progress, remain stable, or regress in utero, a follow-up scan should be performed at 32 weeks for those fetuses in the mildly increased risk category and 4–6 weeks after the initial scan in the moderate to severe category. Postnatal scans are also warranted unless the urinary tract dilation completely resolves prenatally.

Overall, the prognosis for fetal urinary tract dilation is excellent. However, when urinary tract dilation is accompanied by oligohydramnios, the prognosis is worse because the fetus may be born with pulmonary hypoplasia.

Visualization of a small amount of fluid in the renal pelvis is a normal finding on a second- or third-trimester sonogram. With dilation of the renal collecting system, fluid distends the renal pelvis (Figure 14.2.1) and may distend the central or even the peripheral calyces (Figure 14.2.2). The AP dimension of the fetal renal pelvis should be measured so as to help classify the degree of urinary tract dilation as mild (Figure 14.2.3) or moderate to severe
(Figure 14.2.4). A dilated ureter will appear as an anechoic tubular structure extending from the renal pelvis toward the bladder (Figure 14.2.5). When the fetus has a dilated bladder, it will fill the lower abdomen and pelvis and may have a thick wall (Figure 14.2.6). Renal cortical cysts or increased echogenicity, when seen in conjunction with one or more of the above findings, is indicative of parenchymal damage.

When urinary tract dilation is diagnosed, the contralateral kidney should be evaluated to determine whether the process is unilateral or bilateral. The amniotic fluid volume should be assessed, because the fluid volume provides information about urine output and
renal function. A careful search should be made for other fetal anomalies, because the presence of one fetal anomaly increases the risk of other anomalies, and because of the possible association between renal pelvis dilation and trisomy 21. Follow-up ultrasound is important because, while many cases of mild urinary tract dilation resolve during pregnancy (Figure 14.2.7), some cases progress (Figure 14.2.8).

UPJ obstruction, obstruction at the junction between the renal pelvis and the proximal ureter, is the most common cause of urinary tract dilation in the neonate. It is bilateral in 30% of cases, occurs more often in males than females, and affects the left kidney more often than the right. The obstruction is usually partial, and, thus, progression to renal dysplasia is unusual unless the blockage becomes more severe or complete during pregnancy.

The sonographic diagnosis of UPJ obstruction is made when there is dilation of the renal collecting system without dilation of the ureter. It may be either unilateral (Figure 14.3.1) or bilateral (Figure 14.3.2). The amniotic fluid volume is usually normal, unless the obstruction is severe and bilateral. Development of dysplasia in a kidney with UPJ obstruction is uncommon but should be suspected if the renal parenchyma is abnormally echogenic or contains cysts.

Urine normally flows in a unidirectional manner from the kidney through the ureter into the bladder. Ureteral peristalsis propels the urine toward the bladder, and the shallow angle at which the ureter traverses the bladder wall is a configuration that acts as a valve preventing retrograde flow of urine from the bladder back into the ureter.

Two types of problems can arise in the ureter. First, reflux of urine from the bladder into the ureter can occur if the ureter has an abnormally steep, short course through the bladder wall. Vesicoureteral reflux is often bilateral and is more common in males than females. Reflux often resolves spontaneously in utero or within the first 1–2 years of life. Babies born with vesicoureteral reflux are at risk for developing urinary tract infections until the reflux resolves or is surgically corrected. Management after birth often entails prophylactic treatment with antibiotics. Surgical correction is reserved for those cases that are severe at birth or fail to resolve.

The second type of problem that can affect the ureter is an abnormality that interferes with the flow of urine through the ureter. The most common such abnormality is an aperistaltic distal ureteral segment of ureter that causes a functional obstruction. This abnormality, termed primary megaureter, generally has a good prognosis. In mild cases, no intervention is required, and in more severe cases, it is correctable by surgical resection of the affected segment. Webs and strictures are rare ureteral abnormalities that can obstruct flow through the ureter.

The prenatal sonographic findings of vesicoureteral reflux and primary megaureter are similar. Both are characterized by dilation of the renal collecting system and ureter (Figure 14.4.1). Vesicoureteral reflux is often bilateral. In severe cases of primary megaureter, the ureter may
be markedly dilated and tortuous (Figure 14.4.2). Milder cases of reflux or primary megaureter may be misdiagnosed as UPJ obstruction or missed altogether on prenatal ultrasound because the renal and ureteral dilation can be intermittent.

The distinction between vesicoureteral reflux and primary megaureter cannot usually be made in utero, because these two entities have the same sonographic findings. Instead, the specific diagnosis is made after birth, by means of voiding cystourethrography and intravenous pyelography.

Bladder outlet obstruction is most commonly caused by posterior urethral valves that obstruct the urethra, an abnormality that occurs almost exclusively in males. Another cause is urethral atresia, which occurs in both males and females. Prognosis is poor if the obstruction is
complete, because the fetus is likely to develop bilateral renal dysplasia, severe oligohydramnios, and pulmonary hypoplasia. The latter occurs for the same reason that it does in fetuses with bilateral renal agenesis: lack of urinary output leads to severe oligohydramnios and thereby to pressure of the uterine wall on the fetal thorax, restricting pulmonary growth. Renal dysplasia occurs because the kidneys develop in the setting of high output pressure. In some cases of bladder outlet obstruction, the prognosis can be improved with prenatal treatment, such as percutaneous placement of a shunt catheter from the fetal bladder to the amniotic cavity or in utero surgery to ablate the obstructing valves.