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. [https://orcid.org/0000-0002-0736-9199, PMID:16100637]