Antenatal renal pelvis dilatation is considered significant if the anteroposterior diameter is >5 mm in the second trimester or >7 mm in the third trimester and postnatally.
Ultrasound should also note:
- renal size and parenchyma
- ureteric and bladder dilatation
- amniotic fluid volume
Reference: Rennie & Roberton’s Textbook of Neonatology, 5th Ed, 2005. London: Churchill.