The renal pelvis is the part of the kidney that collects urine. Fetal pyelectasis is a prenatal ultrasound finding that is diagnosed with enlargement of the renal pelvis from 4 to 10 millimeters. In many cases, fetal pyelectasis resolves spontaneously. Note that dilatation of the renal pelvis to more than 10 mm is referred to as severe pyelectasis or hydronephrosis. Measurements less than 5mm are normal, should not be designated as pyelectasis, and should not be reported (pmid:16100637).