Antepartum cases of ovarian vein thrombosis are extremely rare and often misdiagnosed. A 22-year-old nullipara at 29 weeks' gestation presented with right lower quadrant pain and fever. Sonography revealed a tubular, hypoechoic mass. At laparotomy, a noninflamed appendix was found and a 2 x 4 cm retroperitoneal mass was palpated near the right ureter. Postoperative magnetic resonance imaging (MRI) revealed bilateral thrombosis of the ovarian veins with caval extension. Therapeutic anticoagulation was achieved, but the patient underwent cesarean delivery 4 days later due to active labor and malpresentation. Postoperative MRI showed no extension of the thrombi, and her postpartum course was otherwise unremarkable. MRI is a valuable, noninvasive tool in the diagnosis of venous thrombi in pregnancy.