Fetal pyelectasis (HP_0010945, a Human Phenotype Ontology entry) can be described as follows. Mild pyelectasis is defined as a hypoechoic spherical or elliptical space within the renal pelvis that measures at least 5mm and not more than 10 mm. The measurement is taken on a transverse section through the fetal renal pelvis using the maximum anterior-to-posterior measurement. Also known as: Fetal renal pelvic dilatation, Foetal pyelectasis, Foetal renal pelvic dilatation, Mild fetal hydronephrosis, Mild foetal hydronephrosis.