Fornix rupture during pregnancy is rare. To be able to initiate the optimal therapy it is important to distinguish the spontaneous fornix rupture from the rupture caused by an obstruction. We report on a patient in the 18th week of pregnancy with strong flank pain on the left side. By means of MR urography the diagnosis of a left fornix rupture due to an obstructive stone could be made. A double-J catheter was placed in the left ureter and the patient received oral antibiotics.