What is extrarenal pelvis on ultrasound scan?

Q: I am in my 32nd week of pregnancy. The recent ultrasound (taken a week back) of my fetus has the following observation - extrarenal pelvis found. Normal variant. The same observation was there in the scan report of the 22nd week. Could you please tell me what this is? Is there anything to be worried about? Is my baby alright?

A:An extrarenal pelvis refers to a normal variant in the baby where the bulk of the collecting system of the kidney is located outside the boundary of the filtration part of the kidney. It is useful, however, to actually measure the anteroposterior diameter of the renal pelvis and the anteroposterior diameter of the kidney to guide further clinical management. If the anteroposterior diameter is less than 10 mm and less than 50% of the renal diameter, then there is no cause for concern.