Benign retroperitoneal teratomas are relatively rare tumors that usually progress slowly and are found in newborns and young adults. Here, we report a case in which a prenatally diagnosed retroperitoneal tumor rapidly increased in diameter from 55 mm to 90 mm during the neonatal period. The tumor was removed while preserving the surrounding organs. Moreover, the patient's postoperative course was uneventful. Pathological examination revealed an immature teratoma with no malignant findings. Notably, even benign neonatal teratomas may exhibit rapid growth of their cystic components after birth.
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