Sacroccygeal teratoma is embryological remnant of:-

Sacrococcygeal teratoma Teratomas occur most frequently in the neonatal period, sacrococcygeal region is the MC site More common in females Thought to be a derivative of the primitive streak Most often an obvious external presacral mass. Most of the tumor is usually external, with a minimal intrapelvic presacral component These lesions should be carefully followed with serial USG until delivery because the blood supply to the tumor may grow to the point of stealing a significant propoion of placental blood flow to the fetus The development of hydrops or placentomegaly is associated with a poor prognosis Most neonatal SCTs are benign Incidence of malignancy is related to age at time of diagnosis and is most frequently represented as yolk sac tumors or embryonal carcinomas. Treatment Complete surgical excision through a chevron-shaped buttock incision. Resection of the cocyx is critical because failure to remove this structure results in significantly higher local recurrence rates.