Though in most cases these stones pass on their own but if not then it can result in many complications like initiation of normal labour, risk of urinary tract infection or urosepsis.Treatment in pregnancy varies individual to individual. For pain relief consult your urologist considering what is safe during pregnancy. Bed rest, hydration, analgesia, stent placement, percutaneous nephrostomy are the possible options. The size of stones, position of stones and your general health condition will help in deciding the exact treatment for you. Hope it helps. Take care and good luck!