To summarize the clinical and ultrasonographic features of retroperitoneal fetus in fetu (FIF) in the prenatal and neonatal period.

Eight cases of retroperitoneal FIF confirmed by pathology from January 2016 to January 2020 at Maternal and Child Health Hospital of Hubei Province were enrolled in this retrospective study. The clinical and ultrasonographic features of retroperitoneal FIF were analyzed and compared with surgical and pathological results.

The eight cases of retroperitoneal FIF were characterized by heterogeneous cystic-solid masses with clear boundaries and thin walls. FIF was typically enclosed within a distinct sac containing either fluid or sebaceous material, ultrasound can identify dysplastic spinal axis, skull ring, long bone, limb bud, thickened skin, and other structures, and the solid components were surrounded by cystic fluid. An umbilical cord-like structure served as the connection between the parasitus and the host. Most of the masses were located slightly to the left or right of the axis of the abdominal cavity. All the eight cases were operated on 5-27 days after birth, including 1 case with ileal Meckel's diverticulum and 1 case with mature cystic teratoma. Four cases underwent laparotomy, one underwent laparoscopic conversion to laparotomy, and three underwent laparoscopic surgery. All the eight cases recovered well after operation, and no recurrence was observed during follow-up. The coincidence rates of ultrasound with pathology were 9/9 for bone tissue, 2/2 for cranium ring, 6/7 for axial skeleton, 6/8 for vertebral column, 6/9 for limb buds, 6/10 for vessels, and 4/10 for skin.

The ultrasonic manifestations of FIF are characteristic and correlated with the clinical and pathological findings. Systematic and standardized ultrasound examination in the prenatal period can diagnose FIF as early as possible, which is of great significance for the selection of appropriate treatment methods and timing after birth.