Mild fetal renal pelvis dilatation: much ado about nothing?

Background: Renal pelvis dilatation (RPD) occurs in 1% of fetuses. Severe RPD (>15 mm) is frequently associated with urinary tract pathology. For the majority with mild (5 to 9 mm) to moderate (10 to 15 mm) RPD, however, there is uncertainty about the risk of abnormalities and how much postnatal investigation is required.