Background. Conventional sonography is the primary imaging tool for these pregnant women who present with an ovarian teratoma. In some cases, however, sonography diagnosis is difficult. We report a case of ovarian teratoma during pregnancy diagnosed by three-dimensional Power Doppler. The cyst was removed via laparotomy without fetal or maternal complications. Three-dimensional ultrasound with multiplanar view can better discriminate a benign ovarian teratoma from complex ovarian lesions or malignant tumors. Its role is significant especially during pregnancy as it may assist in determining which patients are requiring surgery and which are not. The results of three-dimensional sonography and magnetic resonance (MR) were equal but the role of MR imaging is limited in early pregnancy. Conclusions. Three-dimensional technique is a reliable diagnostic modality for preoperative assessment of an ovarian teratoma as it can be performed during the first trimester of pregnancy.