Retroperitoneal cyst is an extremely rare complication of pregnancy. The management of this rare clinical entity is not well understood. An 18-year-old primigravid woman at 11 weeks of gestation with twins presented with complaints of severe nausea and vomiting. Abdominal magnetic resonance imaging showed a 25 x 18 x 10-cm retroperitoneal cyst reaching up to the level of xiphoid processes. No solid component or ascites was seen. She underwent ultrasound-guided percutaneous aspiration of the cyst and 2 L of fluid was removed. The cytology was negative for malignant cells. There was no recurrence of retroperitoneal cyst during the subsequent pregnancy. At 37 (0)/ (7) weeks' gestation, the patient spontaneously delivered the female fetuses in cephalic and breech presentation. There were only seven cases reported in the literature of retroperitoneal cysts during pregnancy between 1955 and 2008. Retroperitoneal cyst during pregnancy is characterized by its extremely rare incidence and its massive cyst size. Because of the difficulty in surgery due to the gravid uterus and close proximity to major organs and blood vessels, percutaneous aspiration of cyst could be an option during pregnancy.