FIF was defined by Willis in 1935 as “a mass containing a vertebral axis often associated with other organs or limbs around this axis” . Since then, the presence of a vertebral axis in a fetiform mass has been considered pathognomonic of FIF. It must be differentiated from a retroperitoneal mature teratoma (RMT) which has an accumulation of pluripotential cells without organogenesis or vertebral segmentation . Some authors consider the two entities as part of a spectrum that includes conjoined symmetric twins; fetus-in-fetu; embryonic vestigial inclusion; and fetiform teratoma .
Demography: The incidence of FIF is 1: 500 000 live births with a slight male preponderance . Besides the retroperitoneum which is the most common site of occurrence, it has also been described in the cranial cavity, oral cavity, mediastinum, lung, sacrococcygeal region, kidneys, intra-abdominal and scrotum [4-6].
Clinical presentation and diagnosis: The clinical presentation is either an incidental mass or with symptoms due to compression of adjoining structures in the abdominal, thoracic or cranial cavities .
Demonstration of metameric segmentation of its spinal axis on imaging modalities usually clinches the diagnosis. However, non-visualization of the vertebral axis on plain film of the abdomen or on CT scan does not exclude FIF, as the non-calcified vertebrae may be radiolucent, but will definitely be demonstrated on histopathology .
Prenatal diagnosis of FIF is possible in about 15% of reported cases (as in our case) . Features suggestive of FIF on imaging have been described as early as 16 weeks of gestation . Prenatal MRI can also be useful. Serum markers like AFP and βhCG may be normal or marginally elevated [8, 9]. The most important differential diagnoses to be considered are teratoma and meconium pseudocyst; both of which will also show calcifications on plain radiography . Besides the above mentioned conditions, intussusception, Wilms’ tumour, neuroblastoma and retroperitoneal lymphadenopathy should also be excluded .
Difference between FIF and RMT: Table 1 delineates the various differences between FIF and RMT. A basic point to remember is that unlike teratoma, FIF is not a true tumour.
Pathology: Detailed and meticulous gross as well as histopathological examination of the excised specimen is the gold standard for diagnosing FIF. The different organs seen in order of frequency are vertebral column (91%), limbs (82.5%), CNS (55.8%), gastrointestinal tract (45%), vessels (40%), and genitourinary tract (26.5%). Hence the vertebral axis may be absent in as much as 9% of cases .
Treatment & follow up: Complete surgical excision is the treatment of choice as malignant recurrence has been reported when the membrane was left behind . For the same reasons, these patients should be kept under close follow up with serial ultrasounds and serum tumour markers. Good surgical acumen is needed as injury to the bile duct has also been described during excision of FIF .
Theories of origin
Included twin theory: This is the most commonly accepted theory of origin of FIF. The supporters of this theory regard FIF as a rare form of monozygotic twinning whereby an aberrant asymmetric twin becomes internalized in the other twin thus acting endoparasitically. Developing from a single ovum, they usually share the same sex, blood group and exactly the same and normal karyotype . This is further strengthened by demonstration of trisomy 21 in both the host as well as FIF by Lee et al .
Supporting this school of thought are Spencer's observations that FIF and teratomas are both part of a spectrum of defective conjoint twinning. Both may coexist and there may be multiple foetuses in FIF. He proposed a primary cardiac malformation in the aetiogenesis of FIF with secondary anencephaly .
Beaudoin's  theory of defective implantation during the second week of development resulting in the invasion of a second embryo (that becomes a homunculus) into the extra-embryonic mesenchyme of the host foetus or autosite, instead of the uterine wall, appears plausible.
The absence of normal umbilical vessels and a definite vascular connection explain the growth retardation and arrest of organ differentiation in almost all cases of FIF .
The presence of an axial skeleton implies development of the included fetus past the primitive streak stage when the notochord is formed, a stage thought to be too developed for formation of a teratoma .
Fetiform teratoma theory: Willis nurtured the theory that teratomas were derived from embryonic pleuripotential cells associated with the primitive streak, which escaped organizer influence to form a true neoplasm, which may later exhibit benign or malignant characteristics . However, concomitant presence of FIF with mature or immature teratoma in several instances implies a common embryological origin . Supporters of this theory believe that FIF represents a well- differentiated, highly-organized or fetiform teratoma .
Association of FIF with ipsilateral atrophic testis: The normal descent of testis is known to occur during late gestation. Any retroperitoneal mass lesion during fetal life can hinder the descent or vascularity of the testis resulting in cryptorchidism or anorchia respectively. Since the FIF in our case was small it affected only the ipsilateral testis. The testicular atrophy could have resulted from extrinsic compression of the testicular vessels by the FIF, since the testicular vessels could not be identified during laparoscopy. Moreover, the FIF may have been larger to begin with producing pressure effects, and regressed later to its small size. Pelvic or large retroperitoneal FIF have been reported to cause bilateral testicular maldescent . However, to the best of our knowledge, testicular atrophy associated with retroperitoneal FIF has not been reported till date in English literature.
To conclude, with improvement in imaging modalities, more cases of FIF are being diagnosed preoperatively and even prenatally (as in our case). The association of FIF with ipsilateral testicular atrophy was also noteworthy.