Teratomas are one of the most common tumors diagnosed in fetuses and newborns. In this age group, extragonadal teratomas predominate, among which the mediastinum ones accounts for up to 15% of teratomas of the perinatal period. They may be associated with the thymus or thyroid gland; in some cases a clear connection with adjacent organs cannot be identified. Teratomas of the heart, also localized in the mediastinum, are often considered separately from the mediastinal ones; most often the tumor affects the pericardium. The article describes two cases of mediastinal teratomas detected by ultrasound in the second trimester of pregnancy, with signs of rapid growth and compression symptoms typical for this localization of the tumor — hydropericardium and other manifestations of non-immune fetal hydrops. In both cases, immature teratomas (grade 2 and 3) were diagnosed with a predominance of immature neuroectodermal tissue, as well as with the presence of endoderm derivatives, including areas of the hepatoid structure, microglandular structures and cysts lined with mucus-forming epithelium. The mesodermal component was represented by hyaline cartilage. An immunohistochemical study revealed an extremely high proliferative activity in the immature neuroectodermal component — more than 90% of positive nuclei were detected in the neuroepithelium upon reaction with Ki67 antibodies. In all other tissue elements, proliferative activity was low.