Purpose or Case Report:
Gastric teratomas are exceedingly rare tumors derived from one or more germ layers. These tumors predominantly occur in sacrococcygeal-gonadal locations but can occasionally manifest in midline regions such as the mediastinum, retroperitoneum, and the head/neck.

Case Presentation: A 1-day-old male infant, born at 36 weeks and 4 days via a cesarean section due to multiple fetal anomalies, including large for gestational age (LGA), new-onset polyhydramnios and significant bilateral hydroceles, presented with a significantly distended and firm abdomen. The initial X-ray revealed extensive amorphous calcifications throughout the abdomen, suggestive of meconium peritonitis. However, a subsequent ultrasound examination revealed a large heterogeneous mass in the upper and mid abdomen, characterized by calcifications, cysts, and soft tissue components. In retrospect, the large mass was detected on the prenatal sonographic exam but misinterpreted as echogenic bowel loops. A subsequent CT scan unveiled a 13.4 x 7.6 x 9.9 cm mass originating from the gastric wall, extending into the gastric lumen as well as the peritoneal cavity.
Interestingly, endoscopy revealed a normal mucosa covering the mass. The tumor was resected, including partial gastrectomy, and histology confirmed the presence of native gastric mucosa and an underlying mass composed of endodermal, ectodermal, and mesodermal tissues, consistent with mature teratomas.

This case report presents a large mature teratoma within the gastric wall, exhibiting exophytic and endoluminal growth, which led to polyhydramnios, abdominal distention, and the development of large hydroceles in the newborn, feeding difficulty. Additionally, it emphasizes the importance of thorough prenatal and postnatal imaging evaluations for timely diagnosis and management.
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