Ultrasound features that might be associated with some type of uropathology include pelvic AP diameter greater than 8 mm, massive dilation of the ureters and renal pelvis bilaterally, increased renal cortical echogenicity, thickened bladder wall, and reduced levels of amniotic fluid. Lee et al. in 2003 showed in a meta-analysis that moderate to severe hydronephrosis had a higher risk of pathology [23]. However, no other specific ultrasound features were found to have any predictive value [22, 24]. The differential diagnosis of fetal hydronephrosis is lengthy and includes obstructive processes such as ureteropelvic or ureterovesical obstruction as well as posterior urethral valves in boys, reflux of urine from the bladder to the renal collecting system, and more uncommon conditions such as prune-belly syndrome. Again, it is important to remember that dilatation of the upper urinary tract is not necessarily due to obstruction and that ultrasound alone is not an accurate test for obstruction as it does assess function or physiology.

The etiology of idiopathic hydronephrosis has not been determined, but the suspicion is that the ureter may have been kinked early in development as urine production starts. The kinking causes a transient obstruction to the flow of urine which will then cause back up of urine into the renal collecting system, thus, causing dilation. With differential growth of the fetus, the ureter may straighten out and the obstructive process may disappear leaving the collecting system dilated to varying degrees.

Marked dilation of the renal collecting system may be associated with high-grade obstruction (Fig. 8.3). If it is bilateral, the obstructive process may be intravesical (posterior urethral valves in males) (Fig. 8.4). As noted above, the more severe the degree of dilation of the renal collecting system, the more likely the chances of pathology [23, 25]. Vesicoureteral reflux has been mentioned as a possible etiologic factor for prenatally diagnosed hydronephrosis and is reported to be present in approximately 12 % of fetuses with ANH independent of the grade of hydronephrosis [23]. The diagnosis of reflux cannot be achieved by prenatal ultrasound but requires postnatal evaluation by VCUG (Fig. 8.5). Suspicion of vesicoureteral reflux can be raised if, while monitoring the bladder and kidneys, upper tract dilation is seen at the time of bladder contraction. This is a rather subtle and unreliable finding but has been described as a possible feature of prenatal vesicoureteral reflux [26]. Concomitant presence of both an obstructive process, as well as reflux, may also be present (Fig. 8.5).

Evaluation of the renal cortex is also an important factor in the survey of the genitourinary tract of the fetus. The echogenicity of the renal cortex should be similar to that of the fetal liver. Increased echogenicity of the renal parenchyma may be associated with renal maldevelopment otherwise known as renal dysplasia [27, 28]. Cystic renal disease of the kidney can also be identified fairly early in gestation by an increase in the echogenicity of the renal cortex (Fig. 8.6). It would be important to differentiate between congenital cystic kidney disease which is bilateral and with a genetic cause (autosomal versus dominant polycystic kidney disease ADPCKD versus ARPCKD) versus multicystic dysplastic kidney (MCDK,) which is usually unilateral with no known genetic etiology. On fetal echography, ARPCKD displays striking features of bilateral, massively enlarged kidneys that are echogenic due to acoustic interfaces caused by the dilated tubules and cysts (Fig. 8.7). Features of multicystic dysplastic kidney as diagnosed prenatally include multiple cysts of variable sizes, no evidence of parenchyma or collecting system, and the possibility of contralateral compensatory hypertrophy (Fig. 8.8). The findings of hyperechoic kidneys in the fetus require postnatal evaluation as it may indicate some intrinsic abnormality of the renal parenchyma, although the outcomes may be quite variable [29]. Confusion may also arise with extrarenal cystic lesions which may mimic renal anomalies. For example, adrenal hemorrhage may give the appearance of an upper pole cyst. Careful, precise scanning can generally differentiate the kidney from the extrarenal lesion. Further diagnostic testing by MRI will provide additional anatomic detail (Fig. 8.9). Other abnormalities of the kidneys include anomalies of position and rotation as well as agenesis or hypoplasia, which are rare.

Any abnormalities of the renal collecting system and/or renal parenchyma should be communicated to the pediatrician who will then decide whether or not further postnatal evaluation is required. Early identification of renal or bladder anomalies warrant prenatal counseling by a pediatric urologist in order to ensure adequate postnatal evaluation and management.

Dilatation of the ureters may also be noted on prenatal ultrasound. If it is unilateral, the dilation is usually associated either with an obstructive process at the distal end of the ureter (ureterovesical junction obstruction or megaureter) or with reflux. Unilateral dilatation of the renal collecting system and ureter does not usually warrant any further intervention during pregnancy but will require postnatal evaluation. Bilateral dilatation of the ureters, on the other hand, warrants careful evaluation of the bladder to look for intravesical obstruction. Other anomalies associated with bilateral dilatation of the renal collecting system and ureters include prune-belly syndrome as well as high-grade vesicoureteral reflux.

Duplication anomalies of the kidneys can be identified later in gestation when the renal parenchyma may be seen to have a band of tissue separating the renal collecting system one or both showing some degree of dilatation (Fig. 8.10). Duplication anomalies diagnosed prenatally should raise the suspicion for vesicoureteral reflux (usually to the lower pole), ureterocele, and ectopic ureters and warrant postnatal evaluation.

Absence of a one or both of the kidneys can be observed prenatally. It is important to ensure that the apparently absent kidney is not ectopic (usually in the lower abdomen or bony pelvis) before making a diagnosis of renal agenesis. Bilateral renal agenesis is rare and occurs in 1–4,800 to 1 in 10,000 births [30]. Unilateral renal agenesis occurs in 1 in 1,100 births. There is usually a male predominance (75 %). There may be an autosomal recessive inheritance associated with this condition. Unilateral renal agenesis should be confirmed postnatally. Bilateral renal agenesis is thought to be incompatible with survival of the fetus and is associated with gradual or sudden drop in amniotic fluid leading to anhydramnios low or inexistent levels of amniotic fluid as the pregnancy progresses leading to fetal crowding and profound lung hypoplasia (Potter’s syndrome) [31].

Renal and suprarenal masses will, occasionally, may be identified prenatally. These include mesoblastic nephroma which is a benign lesion of the kidney. Adrenal hemorrhage may be also identified as a large hypo- or hyperechoic mass above the kidney (Fig. 8.9). Neuroblastoma can also be diagnosed prenatally typically in a suprarenal position but can occasionally be seen in the abdomen or the retroperitoneum.

Postnatal evaluation is dictated by the prenatal findings. No guidelines have been set up for postnatal evaluation of patients diagnosed with hydronephrosis. However, currently it would be reasonable to suggest that any child whose AP diameter of the renal pelvis is <7 mm, probably does not require any follow-up postnatally. A postnatal ultrasound certainly is not an unreasonable approach for patients with an AP diameter of 8 mm or greater. The timing of the first postnatal ultrasound is a matter of debate. An early evaluation (within the first days of life) may allow for early identification of those fetuses who may require follow-up. It is important to realize, however, that sonography during the first 24 h of life may significantly underestimate the degree of hydronephrosis because of physiologic oliguria, so most authorities recommend evaluation after the first few days of life, or later, unless a serious abnormality, requiring urgent treatment, is suspected. Evaluation for vesicoureteral reflux or posterior urethral valves would be in the form of a voiding cystourethrogram (VCUG); for an obstructive process, a MAG 3 radioisotope study would be indicated.