Striking findings concerning the variability in the measurement of the fetal renal collecting system

Objective Using strict thresholds of normality, investigators have reported associations between fetal pyelectasis and aneuploidy, obstructive uropath y, postnatal pyeloplasty and vesicoureteral reflux. Although evidence conti nues to mount regarding the importance of fetal pyelectasis little is known of the variability of collecting system measurements. To investigate the s hort-term variability and its relationship to bladder dilatation, the follo wing study tuns conducted. Design During May and June 1996, 20 mid- and late-trimester patients with v arying degrees of fetal pyelectasis were recruited into this study. Each co nsented to undergo periodic (every 15 min for 2 h) ultrasound measurements of the fetal renal collecting systems and bladder. To account for anatomic and technical variability in measurement, we used both the sum of the trans verse and anterior-posterior (AP) measurements of the collecting systems, a nd AP measurement alone for comparison. The variability of the collecting s ystem measurements was assessed based upon gestational age, magnitude of py electasis and fetal bladder size. Results We investigated 38 renal units in 20 patients (eight in the second trimester and 12 in the third). The mean(SE) AP diameter of all kidneys was 5.89(2.49) mm (range 15 (2-17) mm), and the sum of transverse and AP renal collecting system diameters was 13.91(5.73) mm (range 26 (4-31) mm). These data were normally distributed When assessing the variability in individua l kidneys over time, we found the mean variation (minimum to maximum) for t he sum of the AP and transverse measurement to be 7.61(4.26) mm and for the AP measurement alone to be 3.80(2.49) mm. No relationship was found betwee n variability of dilatation, magnitude of dilatation or fetal bladder size. Cyclic dilatation of the fetal bladder was observed in all cases. The mean time from maximal to minimal dilatation was 20 min (1.34 observations; ran ge 12-30 min). Conclusions We found the size of the fetal renal collecting system to be hi ghly variable over the course of a 2-h period. Seventy per cent of cases (1 4 of 20) had both normal (< 4 mm) and abnormal values (greater than or equa l to 4 mm) during the 2-h study period. Significant caution should be used when considering the implications of renal collecting system dilatation bas ed upon a single AP measurement.