Ovarian teratoma is about 15% of the total number of primary ovarian tumors, and 95% to 98% of them are benign, while only 2% to 5% may be malignant. Ovarian teratoma is a type of ovarian germ cell tumor, also known as ovarian skin-like cyst. The cause of ovarian teratoma is not known, because most of them occur before the maturation and division of oocytes, and it is estimated that they may be caused by the failure of the first maturation and division. 1.Unisexual reproduction theory Unisexual reproduction due to atypical division caused by stimulation of primordial germ cells is the most common etiological theory of immature teratoma. Early studies suggest that teratomas are derived from primitive abnormal tissues in the early embryonic stages with self-differentiating instincts. These leftover “totipotent cells” undergo uncoordinated growth and embryonic developmental disorders, thus detaching from the whole and disordered differentiation, overgrowth, and eventually forming teratoma. The tumor contains ectodermal, mesodermal and endodermal tissues, so the tumor may contain ectodermal tissues such as hair, oil, skin, teeth, bone fragments, etc., and may also contain mesodermal or endodermal tissues such as muscle, gastrointestinal and thyroid tissues. Because the center of ovarian teratoma is often on one side and in a high position, it is easy to be twisted. Clinical manifestations of ovarian teratoma Mature teratoma can occur at any age, mostly between 20-40 years old. When the tumor is small, there are no obvious clinical symptoms, and most of them are found by chance during physical examination. When tumor increases to medium size, abdominal distension or abdominal mass can be found with clear boundary. If the tumor grows to fill the pelvic and abdominal cavity, compression symptoms will appear, such as frequent urination, constipation, shortness of breath and palpitation. About 10% of patients have acute abdominal pain due to tumor rupture, torsion or bleeding. Immature teratoma is mostly seen in young patients with an average age of 11-19 years. Early stage is often asymptomatic, and the main symptoms in late stage are abdominal distension, abdominal mass, ascites and gastrointestinal symptoms. If the tumor infiltrates into the surrounding tissues or compresses the nerves, it may cause abdominal pain, lumbar pain or lower limb pain; if it compresses the pelvic veins, lower limb edema appears. In late stage, it may show cachexia, severe anemia and other cachexia symptoms. Teratoma is divided into two categories: mature type and immature type. 1.Mature type teratoma is mostly cystic, called cystic teratoma, which is benign and mostly unilateral. It is the most common among ovarian germ cell tumors, accounting for 97%-99% of ovarian teratomas and 20% of all ovarian tumors. They occur mostly in women in their reproductive years. Immature type teratoma is less common and accounts for only 1% to 3% of ovarian teratomas. It is mostly seen in young patients under 25 years old.