We report on a woman in her third pregnancy who presented with acute abdominal pain in the 27th week of gestation. After a long car drive she felt an acute pain in the upper right abdomen which moved down to the right lower abdomen. On admission the obstetrical examination showed no pathology. There were no contractions. Clinical and sonographic examination could not reveal the cause of the pain. As adequate analgesia failed to relieve the pain, an explorative laparotomy was performed. It showed a dilated right ovarian vein of 3 cm diameter with a thrombus reaching the confluence to the vena cava. The ovarian vessels were resected and the adnectomy on the right side performed. Anticoagulation was initiated. There were no complications during the postoperative course. The pregnancy proceeded normally. After 10 days the patient could be dismissed in good condition. Laboratory diagnostic showed an deficiency in protein C. At 39 weeks of gestation a planned caesarean section was performed without any complications.