TY - JOUR AB - Fetus in fetu (FIF) is a rare anomaly of diamniotic monochorionic twins, where a malformed fetus resides within the body of its twin. Most FIF occurs in the retroperitoneal region around the host spine and appears prenatally as a solid‑cystic mass consisting of fetal‑like structures. Imaging has an important role in the diagnosis of FIF. The present study reported a single case, a 45‑year‑old woman, with a teratoma in a third‑trimester fetus diagnosed after prenatal ultrasonography (US), which showed a mass containing fetus‑like echoes. FIF was considered after the US showed that the mixed solid‑cystic retroperitoneal mass around the vertebral axis of the host fetus consisted of two separate masses, each containing distinct fetal visceral structures. One fetus was acardiac and the other parasitic fetus was visible with a weak heartbeat. Postpartum magnetic resonance imaging and ultrasonography (US) scans of the newborn showed a retroperitoneal cystic space‑occupying mass with distinctive limbs and visceral structures. The pathological examination further confirmed the diagnosis of retroperitoneal FIF. Also, a prenatal US could detect FIF in utero. A cystic‑solid mass containing long bones, vascular pedicles, or visceral structures around the vertebral axis of the host fetus in the US might suggest the possibility of a FIF. AD - Department of Ultrasound Medicine, Ziyang Hospital of Traditional Chinese Medicine, Ziyang, Sichuan 641300, P.R. China Department of Ultrasound, People's Hospital of Huidong County, Liangshan Yi Autonomous Prefecture, Sichuan 615200, P.R. China AU - Peng,Wentong AU - Zeng,Shaoqin AU - Dong,Mei AU - Peng,Wentong AU - Zeng,Shaoqin AU - Dong,Mei AU - Peng,Wentong AU - Zeng,Shaoqin AU - Dong,Mei DA - 2023/06/01 DO - 10.3892/etm.2023.11983 IS - 6 JO - Exp Ther Med KW - Fetus in fetu fetal abnormalities ultrasonography prenatal case report PY - 2023 SN - 1792-0981 1792-1015 SP - 284 ST - Ultrasound diagnosis of a retroperitoneal fetus in fetu: A case report T2 - Experimental and Therapeutic Medicine TI - Ultrasound diagnosis of a retroperitoneal fetus in fetu: A case report UR - https://doi.org/10.3892/etm.2023.11983 VL - 25 ER -