Final answer:
The most common cause of unilateral fetal hydronephrosis is congenital hydronephrosis.
Step-by-step explanation:
The most common cause of unilateral fetal hydronephrosis is congenital hydronephrosis. This is a condition where there is an obstruction in the flow of urine from the kidney to the bladder on one side.
Some other congenital factors that can cause fetal hydronephrosis include congenital obstruction of the urinary tract, duplicated ureter, horseshoe kidney, polycystic kidney disease, renal dysplasia, unilateral small kidney, and multicystic dysplastic kidney.