To summarize the prenatal ultrasonic characteristics of fetus in fetu (FIF).

The ultrasound images of eight cases of pathologically-confirmed FIF were retrospectively analyzed at Maternal and Child Health Hospital of Hubei Province from January 2009 to January 2019.

Eight cases of FIF were enrolled in this retrospective study. Ultrasonic imaging showed heterogeneous cystic-solid masses with clear boundaries. The locations of masses included the retroperitoneum (4 cases), sacrococcygeal region (2 cases), and face (2 cases). In the 8 patients, 10 cystic-solid masses in fetus were found by ultrasound at different times of gestation from 18^{+ 2} to 38^{+ 1} weeks, with 2 cases having 2 parasites each. The size of masses ranged from the minimum of 1.1 cm×1.1 cm to the maximum of 16.0 cm×10.0 cm. The surrounding organs were compressed differently as a result of the mass effect. Four cases were diagnosed as FIF by prenatal ultrasound, 2 cases were thought to be teratomas, and 2 cases had an unclear diagnosis. The 4 cases with FIF in the retroperitoneum underwent surgery after birth and recovered with no recurrence, and the other 4 cases underwent induced labors. Pathologically, different organ components were found in 10 parasites, including bone/cartilage and limb buds (9/10), vertebral column, skin and vessels (7/10), long bones and hairs (6/10), and intestinal tracts (5/10). The coincidence rates between prenatal ultrasound and pathology were 8/9 cases of bone/cartilage, 6/7 cases of vertebral column, 5/6 cases of axial skeleton, and 5/7 cases of skin and vessels. Prenatal ultrasound failed to detect some organs, including hairs in 6 parasites, intestinal tracts in 5, kidney in 1, and combined malformations in 5 (umbilical bulge in 2 parasites, anencephalus in 2, and spina bififida in 1). One smaller parasite of twin FIF was misdiagnosed as the bony structure of a large parasitic fetus in prenatal ultrasound.

Ultrasonic manifestations of FIF are characteristic and systemic prenatal ultrasound has a very important role in the diagnosis of FIF.