Fetuses in fetu and retroperinatal teratoma have been considered as two distinct entities [1]; Recent literature question the necessity of presence of axial skeleton to be mandatory for the diagnosis of fetus-in–fetu [2]. This entity is differentiated from teratoma by its embryological origin, its unusual location in the retroperitoneal space, and the presence of vertebral organization with limb buds and well developed organ system. In most cases of FIF, only one fetus exists inside the baby, only on extremely rare cases are multiple fetuses found [3]. The surgical removal of the twin fetus is the treatment of choice.

A two days old, full term male newborn born to a Saudi mother G₅P₅ without any pregnancy or familial significant risk factors. The baby delivered by cesarian section due to breech presentation. Upon examination of the baby abdomen was discovered that a firm, irregular and nontender mass was present in the left hypochondrium extending to the umbilical region, genitalia were normal apart from undescended bilateral testis. Conventional X-ray of the abdomen showed multiple dense radioopacities in the left upper quadrant of the abdomen (Figure 1). An ultrasound of the baby abdomen showed a large encysted, hyperechoic heterogenous complex mass. MRI lumbar spine and CT scan of the baby’s abdomen revealed (Figure 2), (Figure 3) an encapsulated cystic peritoneal mass with multiple bony elements of axial skeleton with pelvic girdle, lower extremity buds, ribs and no cranial vault could be appreciated. On laparotomy, a left upper quadrant retroperineal mass in 8x6 cm in size, covered with a sac containing a partially developed fetus with partial differentiation of the limbs, skull, spine and vertebrae (Figure 4). Both testes were felt retroperitonially. Baby did well and discharged home after few days in good general condition. The chromosome analysis confirmed a male chromosome complement without structural anomalies. The tumor marker, Carcinoembryonic antigen (CEA) was normal.

Fetus in fetu was generally considered a highly differential teratom till 1935. Willis et al [1] defined FIF as a separate entity containing some organs, limbs and vertebral column. The phenomenon of FIF has been described at least since the 17th century [4], but the credit with first description of it go to Meckel in 1800 [5]. In review of the literature, about nine percent of cases of FIF, no vertebral skeleton could be identified an imaging or histopathologically. Moreover, more cases reported with hands, phalanges and nail were found but no vertebral column were existed [3],[6]. Most investigators believe that the presence of a developed vertebral column is not a prerequisite for diagnosis [7],[8]. Spencer [9] has suggested that an FIF must have one or more of the following condition:

• Be enclosed within a distinct sac.
• Be partially or completely covered by normal skin.
• Have grossly recognizable anatomic parts.
• Be attached to the host by only a few relatively large blood vessels.
• Either be located immediately adjacent to one of the site of attachment of conjoined twins or be associated with the neutral tube or the gastrointestinal system.

A teratoma, on the other hand, shows lesser organization of microscopically identifiable tissues. Teratoma have a broader attachment site with multiple small blood vessels. The differential diagnosis of a complex mass with calcified components in a fetus or neonate include FIF, teratoma, meconium pseudocyst and neuroblastoma [2]. Our case include most of spencer characteristics of FIF. The normal value of CEA marker, the coexistence of vertebral column and the finding of the testis in the retroperionium, all confirm the diagnosis of FIF in this case. Although the prognosis for FIF is more favorable than for cystic teratoma, the presence of immature elements nevertheless indicates the need for close clinical radiological, serological and tumor marker work-up.

We conclude Fetus in fetu is an extremely rare condition. Currently it is considered as a benign condition; however, controversy continues regarding its future malignant potential or association. When mass compression becomes significant, as in our case, the child or adult can suffer from poor growth or development, infections and even lack of organ function. With surgical treatment, normal anatomy and physiology can be restored and the malignant potential excised. Definitive diagnosis is best made using CT and MRI techniques serology test and tumor marker surveillance and in addition to genetic testing and pathologic reviews, physicians must consider the potential for a minimum of 18 months follow up to avoid a missed malignancy. Surgical outcomes are reportedly good following excision, but further data collection is required for long term results.