Hydronephrosis is the most common abnormality reported on prenatal sonographic screening (Thomas, 1990; Blyth et al., 1993). The vast majority of cases are mild, so-called physiologic hydronephrosis, which are of no clinical significance. Numerous theories have been proposed to try to account for this common finding. In the past, one popular theory was that mild fetal hydronephrosis resulted from changes in maternal hydration. However, Hoddick et al. (1985) demonstrated that the degree of maternal hydration had no significant influence on fetal urinary tract dilation.

These findings were subsequently confirmed by Allen et al. (1987). Other potential causes suggested for mild dilation of the fetal urinary tract include transient obstruction, compression by fetal vessels crossing the ureter, vesicoureteral reflux, and natural kinks and folds in the ureter that may occur during development, hyperfiltration of fetal kidneys, or the influence of metabolic or hormonal factors (Homsy et al., 1986; Najmaldin et al., 1990; Zerrin et al., 1993). The hormonal milieu of the fetus may influence the renal pelvic diameter. Maternal hydronephrosis commonly occurs during pregnancy because of the influence of progesterone, a known smooth muscle relaxant. It has been suggested that maternal progesterone may also be responsible for mild fetal upper urinary tract dilation (Cendron et al., 1994).

Distinguishing physiologic fetal renal pelvic distention from significant or pathologic hydronephrosis is a challenge that requires accurate prenatal sonography and follow-up evaluation. Renal pelvic distention may range in anterior/posterior (A-P) diameter from 3 to 11 mm in up to 18% of normal fetuses studied after 24 weeks of gestation (Hoddick et al., 1985). Because fetal hydronephrosis is so common, Arger et al. (1985) proposed criteria to help distinguish abnormal renal pelvic dilation. They suggested that a pelvic diameter of > 10 mm or a ratio of the A-P pelvic diameter to the A-P renal diameter > 0.5 indicated significant fetal hydronephrosis (Figure 80-1A and B). These criteria were subsequently modified by addition of caliectasis as an additional indicator of significant hydronephrosis (Kleiner et al., 1987). This study suggested that caliectasis might be an even more sensitive and reliable indicator for predicting pathologic hydronephrosis than simple pelviectasis (Figure 80-1A). Renal pelvic dilation less than these criteria for pathologic hydronephrosis is considered minimal fetal hydronephrosis. Morin et al. (1996) defined minimal hydronephrosis as renal pelvic dilation > 4 mm but < 10 mm in a fetus that was less than 24 weeks of gestation.

Classification of the degree of renal pelvic dilation is also dependent upon when during gestation it is diagnosed. Fetuses at 15 to 20 weeks of gestation with a renal pelvic diameter of 4–7 mm are considered to have minimal fetal hydronephrosis, while those fetuses with A-P diameter greater than 7 mm are classified as having moderate hydronephrosis. In contrast, after 30 weeks of gestation, the threshold for defining mild, moderate, and severe hydronephrosis is defined as 5–8 mm, 9–15 mm, and over 15 mm, respectively (Pates and Dashe, 2006).

Antenatal hydronephrosis is a common finding on antenatal ultrasound with incidence reported from 0.3% to 4.5% with most reports around 1% (Havutcu et al., 2002). When a number of clinical studies are pooled, the calculated incidence of detectable dilation of the fetal urinary tract approaches 1 in 100 pregnancies (Thomas, 1990). However, difficulties in assessing the true incidence of pathologic fetal hydronephrosis stem from the high incidence of physiologic hydronephrosis and the limitations of criteria used to define pathologic urinary tract dilation (Gruppe, 1987). The overall incidence of congenital hydronephrosis in a large-scale maternal–fetal screening program in Sweden was 0.17% (Helin and Persson, 1986). This figure was lower than the one reported (0.76%) in Britain, in a well-designed prospective study using antenatal ultrasonography at a specific time during pregnancy (Livero et al., 1989). In many of these cases, however, a large number of the fetuses displayed what would be considered physiologic hydronephrosis, in other words, minimal pyelectasis. Follow-up studies in patients diagnosed with prenatal hydronephrosis showed that only 1 in 500 fetuses required prenatal or postnatal intervention for hydronephrosis (Thomas, 1990).

The role of ultrasound examination in the evaluation of the fetal urinary tract is twofold: to monitor the extent of fetal urinary tract dilation and to identify fetuses affected with such severe lesions that intervention or termination of the pregnancy should be considered. Prenatal sonography has irrevocably changed the approach to congenital malformations, particularly those involving the urinary tract (Harrison et al., 1982; Golbus et al., 1983; Mahoney et al., 1984; Harrison and Filly, 1991; D’Alton and DeCherney, 1993). Current diagnostic capabilities allow for the detection of urinary tract anomalies as early as 12 to 14 weeks of gestation (Glazer et al., 1982; Bronshtein et al., 1990; Patten et al., 1990; Blyth et al., 1993). Variables to be considered in the evaluation of fetal hydronephrosis include gestational age at diagnosis, area of urinary tract involved, degree of dilation, and evidence of significant obstruction. Several series have reviewed the accuracy of the diagnosis of hydronephrosis by ultrasound examination of the fetus (Blane et al., 1983; Avni et al., 1985; Watson et al., 1988). Unfortunately, a fairly high rate of false-positive results have been noted, varying between 9% and 22% (Scott and Renwick, 1987; Reznick et al., 1988). It is thought that as our ability to distinguish physiologic from pathologic hydronephrosis becomes more refined, this rate of false-positive scans will be reduced. In order to better define the severity of hydronephrosis, the Society of Fetal Urology has proposed a grading system that uses two parameters: 1) the central renal complex; and 2) renal parenchymal thickness to grade density of the hydronephrosis (see Table 80-1). Most cases of minimal hydronephrosis will fall in the Grade 0 or 1 categories.

Most authors recommend that all fetuses with an A-P diameter of >6 mm should undergo postnatal evaluation (Odibo et al., 2004; Becker and Baum, 2006; Belarmino and Kogan, 2006). Certainly fetuses with an A-P pelvic diameter 10 mm and those who have an A-P pelvic renal cortex ratio >0.5 or with evidence of caliectasis should certainly undergo postnatal evaluation (Cendron et al., 1994).

The differential diagnosis of minimal fetal hydronephrosis includes extrarenal pelvis, prominent renal veins crossing over the renal pelvis, and vesicoureteral reflux (Table 80-2). The degree of dilation in an extrarenal pelvis can be sufficient to be mistaken for ureteropelvic junction (UPJ) obstruction (see Chapter 81). Color flow Doppler studies should be able to distinguish the crossing renal vein from the true renal pelvis. In vesicoureteral reflux the degree of pelvic dilation will vary over time. In particular, increased renal pelvic diameter may be observed at the time of bladder contractions. Other possibilities include an overlying loop of intestine mistaken for the renal pelvis, a dilated superior pole of a duplex collecting system, and dilated ureter secondary to ureterovesical obstruction. In each of these cases careful sonographic examination should be able to define the fetal anatomy.