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.Study design: Systematic review of cohort studies of fetuses with RPD < 15 mm and metaregression to estimate risks of postnatal RPD, obstruction, and VUR.Results: Of 506 potentially relevant papers, 18 met the inclusion criteria. Risk of postnatal RPD increased with fetal RP size and earlier gestation. Odds ratios for postnatal RPD doubled per millimeter increase in fetal RP size: At 20 wk gestation, for example, 18% of fetuses with mean RP of 6 mm were estimated to have persistent postnatal RPD, compared with 95% of fetuses with 12 mm RPD, but risks were decreased by 16% to 18% per week of presentation gestation. Estimated risks of obstruction and VUR were substantially lower, particularly in the mild group such as the 6 mm example above: obstruction 2%, VUR 4%.Conclusions: Our novel risk estimates are useful for antenatal counseling at presentation. The low frequency of obstruction/ VUR in mild RPD raises questions over the most appropriate investigation of these cases but further data are required before establishing definitive postnatal management pathways. We suggest the need for a large prospective multicenter study to collect individual patient parameters/results and search for additional prognostic indicators.