Hi Thanks for posting on HCM Renal hypoplasia is characterized by congenitally small kidneys with reduced number of nephrons.It may be bilateral or unilateral.Unilateral renal hypoplasia may be asymptomatic and is diagnosed during routine ultrasound screening performed during pregnancy or for UTI in children.In severe cases of unilateral hypoplasia, the individual is comparable to some one having only one functioning kidney.This always results in the overactivity of the other kidney and may explain the mild dilatation of the left pelvicaalyceal system in your daughter. Possible causes may include: intrauterine growth restriction, maternal diseases like diabetes and hypertension, maternal drug intake during pregnancy, intoxication with alcohol and smoking, premature birth. I will suggest you see a nephrologist for better management. Hope my answer will help you Best regards