The fetal kidneys begin developing within the pelvis at approximately 7 weeks of gestation from the metanephric mesoderm and the ureteric bud. The metanephric tissue develops into the nephrons of the kidney, and the ureteric bud differentiates into the collecting tubules, calyces, pelvis, and ureter. Between 7 and 11 weeks, as the fetal body grows in length, the kidneys ascend to their permanent position in the flank.¹

Initially, the kidney is made up of several loosely connected lobes, each with a thin cortex. During the second trimester the lobes fuse, becoming less distinct, and the cortex thickens, leaving the kidney with a lobular contour that persists for several years after birth.^1,2 The kidneys begin to excrete urine at approximately 10 to 11 weeks of gestation, and from this age onward urine production increases progressively.

From the second trimester onward, the kidneys are the major contributor to the amniotic fluid volume.^3,4,5 Adequate amniotic fluid volume is necessary for normal fetal pulmonary and skeletal development because it provides space for fetal growth and movement. Therefore, a functioning urinary tract must be present for the lungs and skeleton to develop normally.^5-8

Using transvaginal ultrasound, normal fetal kidneys can first be seen as early as 9 weeks of gestation and are generally visible by the end of the first trimester.⁹ With transabdominal scanning, the kidneys may first be visible at 12 to 14 weeks of gestation and are seen in most patients by 16 to 18 weeks (Figure 22-1).¹⁰ Delayed visualization may occur when factors such as maternal obesity or large uterine fibroids limit the sonographic assessment of fetal anatomy. The fetal bladder may be seen as early as 10 weeks of gestation with transvaginal scanning and should be visible transabdominally by 14 weeks in virtually all patients.^10-12

The incidence of genitourinary anomalies at birth is 0.5% to 0.8%.^13-17 Genitourinary anomalies comprise approximately one-fourth of all congenital structural anomalies.¹⁷ Anomalies of the genitourinary tract can result from a number of causes, including arrested development early in organogenesis, failure of normal ascent, obstruction of the collecting system or bladder outlet, and abnormal formation of renal tubules. Genitourinary tract anomalies are not only isolated, but may also occur in association with, or may cause, other fetal structural abnormalities. Associations between genitourinary and other organ system anomalies occur in a broad variety of inherited or sporadic syndromes, including chromosomal abnormalities.^4,6,13,18-20 In addition, urinary tract abnormalities that decrease urine production cause oligohydramnios, which may secondarily cause deformities involving other parts of the fetus. In particular, when severe prolonged oligohydramnios begins before 20 weeks of gestation, pulmonary hypoplasia, facial abnormalities (including flattened nose and low-set ears), and clubfeet or other limb positional abnormalities may result, findings termed Potter syndrome.^20,21

Prenatal sonography can identify and characterize many anomalies of the genitourinary tract. Ultrasound performed in the second and third trimesters can detect at least 80% to 85% of genitourinary anomalies, although detection rates are lower when the genitourinary anomaly is in a fetus with multiple anomalies.^13,14,16,22 Prenatal diagnosis permits antenatal counseling and prompt postnatal evaluation. This is especially important for anomalies, such as reflux or partial obstruction, in which early postnatal treatment can prevent impairment or loss of renal function.²³

Failure of development of the ureteric bud will lead to renal agenesis. Renal agenesis may be unilateral or bilateral. Unilateral agenesis occurs in approximately 0.3 per 1000 births and has an excellent prognosis. On prenatal ultrasound no kidney is seen in the renal fossa on the affected side, and the contralateral kidney is compensatorily large for gestational age.²⁴ Color Doppler interrogation of the abdominal aorta will demonstrate a renal artery on one side and no renal artery on the other. It is important not to mistake the ipsilateral adrenal, which may be flattened or “lying down,” for a kidney (Figure 22-2).²⁵ This error is avoided by recognizing that the adrenal gland does not have central sinus echoes and does not have a reniform shape in the longitudinal plane. It is also important to scan the fetal pelvis to be sure the kidney is truly absent rather than in an ectopic location.^26,27

Bilateral renal agenesis is a lethal anomaly in which there is failure of development of both kidneys. This condition occurs in approximately 1 to 4 per 10,000 births, with a male to female ratio of 2.5 to 1. When this anomaly is present, there is severe oligohydramnios from the early to mid-second trimester onward, leading to characteristic pulmonary, facial, and limb abnormalities termed Potter syndrome.^20,21 The neonate with bilateral agenesis typically dies shortly after birth from pulmonary hypoplasia. Recurrence in subsequent pregnancies is rare.

Sonographic diagnosis of bilateral renal agenesis is made on the basis of severe oligohydramnios and nonvisualization of the kidneys and urinary bladder (Figure 22-3). Careful scanning is required before making the diagnosis because absence of surrounding amniotic fluid degrades the sonographic image. Other sonographic findings often seen with bilateral renal agenesis include dolichocephaly and a small thorax, both of which are secondary to uterine compression from the absence of amniotic fluid.^28,29

If one or both kidneys develop but fail to ascend normally into the renal fossa, one or both will ultimately reside in an ectopic location. This anomaly occurs in about 1 per 1200 births. The most common ectopic location is in the pelvis. Less commonly, a horseshoe kidney forms, in which the two kidneys are fused at their lower poles and ascend partially, such that their axes are altered and their lower poles extend across the midline of the lower abdomen. Occasionally, one kidney is cross-fused to the lower pole of the contralateral kidney. Other ectopic renal locations, such as in the thorax, are extremely rare.^26-28

The sonographic findings with a pelvic kidney include visualization of a kidney in the pelvis adjacent to the urinary bladder and a renal fossa that is either empty or contains a flattened adrenal gland (Figure 22-4).^26-28 The contralateral kidney will be normal in size for gestational age, without compensatory hypertrophy as may be seen with unilateral renal agenesis.²⁴ A pelvic kidney is at increased risk for urinary obstruction, and, therefore, renal collecting system dilation may be present.³⁰

With cross-fused ectopia, one renal fossa will be empty and the contralateral kidney will appear very long or may be L-shaped (Figure 22-5).²⁸ The lower moiety may have a dilated collecting system or cystic dysplasia secondary to obstruction.³¹

With horseshoe kidneys, the fused kidneys have a U-shape with renal parenchyma visible extending across the midline in front of the fetal aorta and inferior vena cava (Figure 22-6).²⁸ These kidneys also have an increased incidence of obstruction and may have cystic dysplasia.³²

Dilation of the fetal renal collecting system (sometimes termed “hydronephrosis”) is the most common fetal abnormality detected by antenatal sonography. It can result from obstruction of the urinary tract, vesicoureteral reflux, or deficient musculature in the walls of the urinary tract and abdomen (“prune belly syndrome”). Urinary tract obstruction not only occurs at the level of the ureteropelvic junction, but may also occur in the ureter, at the ureterovesical junction, or at the bladder outlet.³³

When fluid is seen in the intrarenal collecting system by ultrasound in the second and third trimesters, it is important to address three questions. First, does the fluid represent renal dilation or is it merely the small amount of fluid often seen in the normal renal pelvis? Second, are any other urinary tract abnormalities, including peripheral calyceal dilation, parenchymal abnormalities (thinning, increased echogenicity, or cysts), ureteral dilation, or distended bladder, present, or is there unexplained oligohydramios? Third, based on the urinary tract findings, what is the risk of postnatal uropathy requiring medical or surgical intervention for urinary obstruction or vesicoureteral reflux?

A key sonographic feature that helps address these questions is the anteroposterior (AP) measurement of the fluid in the fetal renal pelvis on a transverse view through the fetal abdomen. If this measurement is less than 4.0 mm at 16 to 27.9 weeks or less than 7.0 mm at 28 weeks or greater, the fluid should be considered normal and the diagnosis of urinary tract dilation should not be made.³⁴

If the measurement is greater than or equal to these values, the sonogram should be reported as demonstrating urinary tract dilation and the level of risk of postnatal uropathy should be assessed based on the criteria developed at a consensus conference held in 2014 (Table 22-1)³⁴ as follows. The risk of postnatal uropathy is mildly increased if the AP measurement of the fluid in the renal pelvis is 4.0 to 6.9 mm at 16 to 27.9 weeks or 7.0 to 9.9 mm at 28 weeks or greater, and no other urinary tract abnormalities are seen (Figure 22-7). The risk is moderate to severe if either: (1) the AP measurement is at least 7.0 mm at 16 to 27.9 weeks or 10 mm at 28 weeks or above; or (2) one or more other urinary tract abnormalities are present, including peripheral calyceal dilation, abnormal renal parenchyma (thin, echogenic, or cystic), dilated ureters, distended bladder, or unexplained oligohydramios (Figure 22-8).^33-35

The stratification of risk is based in large part on the AP diameter of the renal pelvis because it has been shown that the larger the AP diameter, the more likely it is that the fetus will need postnatal medical or surgical treatment. When the AP measurement falls in the mildly increased risk category, approximately 50% will have urinary tract dilation on the first postnatal ultrasound, but fewer than 5% will require surgery for vesicoureteral reflux or urinary obstruction after birth. Among those fetuses with AP measurements in the moderate to high-risk group, the majority will have significant urinary tract dilation postnatally, and most will require medical or surgical intervention for urinary obstruction or vesicoureteral reflux.^33,36-50

When urinary tract dilation is diagnosed, it is important to examine the contralateral kidney because renal anomalies are often bilateral. In addition, a careful search should be made for other fetal anomalies.²⁸ When mild urinary tract dilation is diagnosed in the second trimester, the possibility of a chromosomal abnormality, especially trisomy 21, must be entertained. Up to one-fourth of fetuses with trisomy 21 have mildly dilated renal pelvises in the second trimester, as compared with 2% to 3% of normal fetuses. It should be noted, however, that while the presence of mildly dilated renal pelvises indicates a higher than average risk of trisomy 21, the large majority of fetuses with renal pelvis dilation and no other abnormalities seen on ultrasound have normal chromosomes. The likelihood of a chromosomal abnormality is especially high when, in addition to urinary tract dilation, the femur is small or there is a cardiac or gastrointestinal abnormality.^34,35,51,52

When mild urinary tract dilation is diagnosed before 28 weeks gestation, follow-up scan at 32 weeks or greater is recommended to determine if the dilation has resolved or progressed.^34,35 When a diagnosis of moderate to severe urinary tract dilation is made on prenatal sonogram, follow-up sonogram in 4 to 6 weeks is recommended. In either case, with mild urinary tract dilation or moderate to severe urinary tract dilation, a postnatal ultrasound is recommended, to be performed during the first month of life but at least 2 days after birth.^34,35

Ureteropelvic Junction Obstruction

Ureteropelvic junction (UPJ) obstruction is the most common cause of urinary obstruction in the neonate and affects males more than twice as often as females. It is bilateral in 30% of cases. On ultrasound, a dilated renal pelvis is seen, with or without dilated peripheral calyces (Figure 22-9). The ureters are not dilated, and the amniotic fluid is usually normal except in rare cases of severe bilateral UPJ obstruction.^{23,33,42,53-55}

Most kidneys with UPJ obstruction do not become dysplastic as a result of the obstruction. Dysplasia does occur, however, in a minority of cases, typically those with severe or early-onset obstruction.^23,42,53 When dysplasia does occur, it is most often the type of dysplasia seen with delayed or incomplete obstruction, although it may evolve into the multicystic dysplastic variety. Development of dysplasia in a kidney with UPJ obstruction can be diagnosed when the parenchyma becomes abnormally echogenic or cystic.

Vesicoureteral Reflux

Vesicoureteral reflux, or reflux from the bladder into the ureter, results from an abnormal relationship between the distal ureter and the bladder wall. It is the second most common cause of urinary tract dilation in the fetus.³³ Normally, the ureter traverses the bladder wall at a shallow angle, leading to a long intramural segment of ureter that acts as a valve preventing reflux. When the ureter has a steep, short course through the bladder wall, reflux may occur. This abnormality is much more common among males than females and is often bilateral. It often resolves spontaneously within the first 1 to 2 years of life. Reflux that does not resolve spontaneously or is very severe at birth can be corrected surgically.^38,39,56

The sonographic findings of vesicoureteral reflux include dilated renal collecting system and dilated ureter (Figure 22-10).²⁸ In some cases, the dilation of the kidney and ureter is present intermittently, with emptying and refilling every few minutes. In severe cases, the ureter may be markedly dilated and tortuous. Care must be taken not to mistake such a ureter for a dilated bowel loop. This error can be avoided by following the dilated ureter proximally to the renal pelvis and distally to the bladder.

Primary Megaureter (also called “Ureterovesical Junction Obstruction”)

Primary megaureter is dilation of the ureter, usually resulting from an aperistaltic distal ureteral segment that produces a functional obstruction. The prognosis is good. In mild cases no intervention is required, and in more severe cases corrective surgery can be performed.^57,58

The sonographic findings are dilated renal collecting system and dilated ureter, similar to findings with vesicoureteral reflux. With primary megaureter, the dilated ureter may be quite large, filling much of the lower abdomen, and may demonstrate peristalsis.²⁸ Because the combination of dilated renal collecting system and dilated ureter is seen with both reflux and megaureter, the distinction between the two usually cannot be made in utero but must await postnatal evaluation.