To investigate the ultrasound and magnetic resonance imaging (MRI) characteristics of retroperitoneal fetus-in-fetu.

The prenatal ultrasound and MRI images of 6 cases of retroperitoneal fetus-in-fetu diagnosed at the Maternal and Child Health Hospital of Hubei Province from October 2016 to January 2020 were retrospectively analyzed, and the findings were compared with the postnatal computed tomography (CT), surgical, and pathological results.

The size of the masses at the first prenatal ultrasound diagnosis ranged from 11 mm×10 mm×11 mm to 40 mm×37 mm×25 mm, and the gestational age ranged from 22⁺² weeks to 34⁺¹ weeks. MRI was performed at 22⁺³ weeks to 40⁺⁵ weeks of gestational age. The mass size ranged from 54 mm×45 mm×38 mm to 125 mm×112 mm×83 mm on ultrasound during 1-12 days after birth. All the 6 cases were accurately diagnosed as retroperitoneal fetus-in-fetu before delivery, and the earliest diagnosis was made at 28⁺⁴ weeks of gestational age. Regarding clinical outcome, all the 6 cases underwent elective surgery during 5-16 days after birth, of which 3 underwent open surgery, and the other 3 underwent laparoscopic minimally invasive surgery, one of whom was converted to open surgery due to difficulties of laparoscopic mass separation. There was no recurrence during follow-up. With regard to pathological results, there were 6 cases of retroperitoneal parasitic fetuses, 4 cases of single parasites, and 2 cases of twin parasites; one case was associated with mature teratoma. Using the pathological results as the golden standard, the highest detection rate of prenatal ultrasound occurred in the bone tissue and skull ring, followed by long bones, spinal shafts, limb buds, vascular pedicles, and skin.

Ultrasound combined with MR and CT as multimodal imaging is helpful to improve the diagnosis of fetal retroperitoneal fetus-in-fetu.