Congenital hydronephrosis is frequently amenable to prenatal diagnosis, often as early as the second trimester. Most clinicians use a renal pelvis anterior-posterior (AP) diameter of 4 mm or more prior to 20 weeks of gestation as a threshold for identifying pyelectasis. If mild dilation of the renal pelvis is an isolated finding in the second trimester, evaluation performed later in gestation is used to guide postnatal management. Since the normal renal pelvis dimensions may increase with advancing gestation, thresholds for the diagnosis are larger in the third trimester. Neonatal follow-up is typically recommended only if the fetal renal pelvis diameter exceeds a specified cut-off (e.g. 7 or 10 mm) at or beyond 34 weeks. If the measurement is less, most deem the pyelectasis physiologic or normal. However, it has been suggested that fetuses with early renal pelvis dilation that resolved during pregnancy might also benefit from postnatal surveillance. The newborn evaluation for hydronephrosis may be time consuming, invasive, and costly; however, it can often prevent sequelae from congenital uropathy.