There are many possible causes for fetal hydronephrosis. Anything that causes a blockage of the urinary tract on that side can be responsible. These are usually easily corrected by surgery after birth, and early delivery by cesarean section is rarely helpful. Unless there are other reasons for a c-section, this is not an indication for a c-section. The most common, in male babies, is something called posterior urethral valves. A full exam will certainly be done when your baby is born, and the doctors will decide what needs to be done. some of the conditions don t even require surgical intervention. While your concern is normal and understandable, i am sure things will be fine. Wishing you the best, please feel free to contact me if you have any more questions.