Renal pelvis is a part of the collection system of our
kidney.
The normal renal pelvis diameter of fetus is less than 7mm before 24 weeks.
So in your case for the fetus to have a renal pelvis diameters of 4.1 and 3.5 is quite normal, in my opinion.
You probably shouldn't worry much about it.
Next Steps
Relax yourself.
Health Tips
Further during this trimester it is usually found to be mildly dilated which resolves by its own by a follow up scan during the 3rd trimester.