FIGURE 10-41 Renal pelvis dilatation. This common finding is identified in 1 to 5 percent of pregnancies. A.oln this 34-week fetus with mild renal pelvis dilatation, the anterior-posterior diameter of the renal pelvis measured 7 mm in the transverse plane. B. Sagittal image of the kidney in a 32-week fetus with severe renal pelvis dilatation secondary to ureteropelvic junction obstruction. The arrow points to one of the rounded calyces.
This finding is present in 1 to 5 percent of fetuses. It is also called urinary tract dilatation or hydronephrosis. In 40 to 90 percent of cases, it is transient or physiological and does not represent an underlying abnormality (Ismaili, 2003; Nguyen, 2010). In approximately a third of cases, a urinary tract abnormality is conirmed in the neonatal period. Of these, ureteropelvic junction (UP]) obstruction and vesicoureteral relux UR) are the most frequent.
he fetal renal pelvis is measured anterior to posterior in a transverse plane, and calipers are placed on the inner border of the fluid collection (Fig. 10-41). Although various thresholds have been deined, the pelvis is typically considered dilated if it exceeds 4 mm in the second trimester or 7 mm at approximately 32 weeks' gestation (Reddy, 2014). Typically, the second-trimester threshold is used to identiy pregnancies that warrant subsequent third-trimester evaluation.
he Society for Fetal Urology categorized renal pelvis dilatation based on a metaanalysis of more than 100,000 screened pregnancies (Table 10-10) (Lee, 2006; Nguyen, 2010). he degree of dilatation correlates with the likelihood of an underlying abnormality. Other suggestive indings of pathology
TABLE 10-10. Risk for Postnatal Urinary Abnormality According to Degree of Renal Pelvis Dilatationa
Second Third Postnatal Dilatation Trimester Abnormality 4 to <7 mm 7 to <9 mm
Moderate 7 to �1 0 mm 9 to �15 mm 45% aSociety for Fetal Urology Classification. Modified from Lee, 2006; Nguyen, 2010.
include calyceal dilatation, cortical thinning, or dilatation elsewhere along the urinary tract. Mild pyelectasis in the second trimester is associated with a slightly greater risk for Down syndrome and is considered a soft marker for this (Fig. 14-3, p.i287).
Ureteropelvic Junction Obstruction. This condition is the most common abnormality associated with renal pelvis dilatation. The birth prevalence is 1 in 1000 to 2000, and males are afected three times more often than females (Williams, 2007; Woodward, 2002). Obstruction is generally functional rather than anatomical, and it is bilateral in up to a fourth of cases. he likelihood of ureteropelvic junction obstruction rises from 5 percent with mild renal pelvis dilatation to more than 50 percent with severe dilatation (Lee, 2006).