Variability in dilatation of the fetal renal pelvis during a bladder filling cycle

To investigate the variation in the dimensions of the fetal renal pelvis in relation to the degree of bladder filling in fetuses with mild pyelectasis.

Eighteen third-trimester pregnant women with mild uni- or bilateral fetal pyelectasis, defined as an anteroposterior (A-P) diameter of the renal pelvis between 5 and 10 mm, were recruited for the study. The women were examined for 2-3 h by ultrasound. The A-P and transverse dilatation of the renal pelvis and the bladder dimensions (to calculate fetal bladder volume) were measured at 2-3-min intervals. Postnatally, all infants were investigated by ultrasound at 3-4 months.

In 6/18 fetuses a consistent relationship between the size of the renal pelvis and bladder filling was found, with a mean difference in renal pelvic diameter before and after voiding of 6.7 mm and a largest observed difference of 14.3 mm. In 12/18 fetuses no such relationship was found. Postnatally, five infants were referred to a pediatric urologist. The investigations in these five infants could not confirm the hypothesis that variation in renal pelvic size in relation to bladder size may predict prenatal vesicoureteric reflux (VUR).

In mild pyelectasis the size of the renal pelvis is highly variable in one-third of cases. The association with bladder volume and micturition suggests evidence of VUR, but this could not be proven. If cut-off values are used to differentiate between normal and abnormal renal pelvic size then not only gestational age but also the degree of bladder filling at the time of measurement should be taken into account. Caution should be expressed when the diagnosis of a possible urological anomaly is based on a single measurement during just one investigation.