Sonographic features of congenital infantile fibrosarcoma that appeared as a sacrococcygeal teratoma during pregnancy

We experienced an extremely rare case of congenital infantile fibrosarcoma originating from the fetal sacrococcygeal region in pregnancy. At first, we suspected fetal sacrococcygeal teratoma; however, the following ultrasonography findings of the tumor complicated this diagnosis: (i) laterality; (ii) no cystic component; (iii) hypervascularity, with the feeding vessels not derived from the middle sacral artery; and (iv) a skin covering. The pathological findings indicated that the tumor was an infantile fibrosarcoma, not a teratoma. Thus, we believe that these sonographic features may aid the prenatal diagnosis of congenital infantile fibrosarcoma, which, in turn, may contribute to a better prognosis and may be useful for parental counseling.