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Fetus in Fetu or Teratoma: A Report of Four Cases and Literature Review

Iftikhar Ahmad Jan, Basmah Al Hamoudi, Murthi Shiniwas

Department of Pediatric Surgery, Zayed Military Hospital, Abu Dhabi, United Arab Emirates



Iftikhar Ahmad Jan

Department of Pediatric Surgery

Zayed Military Hospital, Abu Dhabi, UAE

Cell: 00971552895380

E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.




We reviewed our four cases labeled as fetus in fetu with the purpose to understand the gross, radiological and histological appearance but also the outcome of these masses.


The diagnostic criteria included a teratomatous mass containing axial skeleton that was confirmed by radiologic examination of the mass.


Of the four patients, three were female and one male. Two babies were three months of age, one 18 months and one 5 years. The masses were located in the retro-peritoneum in two cases, one was sacrococcygeal and one was presacral also attached to the coccyx. Complete excision was possible in all the patients and all were symptoms free two year after surgery with no recurrence. The histopathology of the masses showed derivatives from various embryological germ layers.


The initial proposition that fetus in fetu is a parasitic twin may be true in cases of retroperitoneal masses however the presence of fetus in fetu in the sacrococygeal area supports the theory that it develops form of mature teratoma. The important fact is that fetus in fetu offers a cure in most cases after complete resection.

Keywords: fetus in fetu, teratoma, parasitic twins, duplication



Fetus in fetu (FIF) is a terminology used for a tertomatous mass, which shows similarity to a developing fetus. Both fetus in fetu and mature teratoma contain tissue from all three germ layers. The differentiating points from a mature teratoma are presence of axial skeleton and limb buds [1]. The true embryogenesis of FIF is not adequately understood. One theory describes FIF an incompletely developed twin in the body of normally developed baby and is considered as diamniotic, monochorionic and monozygotic twin [2]. The other theory suggests FIF a mature teratoma with mature tissue from different germ layers [3]. The diagnosis of FIF has important clinical implications as most FIF after resection can ensure a permanent cure. We reviewed four cases of FIF with a view to understand the gross, radiological and histological appearance but also the outcome of these teratomatous masses.

Case Reports

Case 1

A 5 years girl was referred to us with the history of, recurrent urinary tract infections, urinary incontinence and possible neuropathic bladder. The child had flattening of lower sacral spine suggesting spina bifida occulta. Ultrasonography and MRI showed a large presacral mass attached to the coccyx suggesting a type-IV sacrococcygeal teratoma, Excision of the mass was performed through posterior sagittal approach and a 12x10 cm mass was excised along with coccygectomy (Fig. 1).

Figure 1: Presacral mass showing partially developed spine, pelvis and other body skeleton.


X-ray of the mass showed the par- tially developed axial skeleton, pelvis structures and also irregular bony developments suggesting rudimentary limb. Histopathology confirmed tissue from various germ layers including renal, neural, gastro-intestinal,muscles, bone and cartilage. The child had uneventful recovery and was symptoms free at two years follow-up.

Case 2

An 18 months girl was referred to us with the diagnosis of a mass in the right abdomen incidentally noticed by the grandmother. Ultrasound and CT scan of the abdomen suggested a multicystic mass in right side of the abdomen, with calcifications and compression of the right kidney. Excision of the mass was performed through right transverse abdominal approach. Complete excision of the mass was performed (Fig. 2).

Figure 2: Retroperitoneal fetus in fetu with partial formation of axial skeleton, rudimentary limb buds and pelvis


X-ray showed the pelvic and spinal structures suggesting a fetus in fetu. Histopathological exam showed mixture of mature tissue including skin, hair follicles, sweat glands, glial tissue and respiratory mucosa. The child had uneventful recovery after surgery and was symptoms free 2 years after surgery.

Case 3

A three months boy presented with a mass in the abdomen. Ultrasonography and CT scan suggested a thoraco-abdominal mass pushing the esophagus (Fig. 3). It was possible to excise the mass through an upper abdominal transverse incision. The mass was bi-lobulated and suggestive of an fetus in fetu. On gross examination the mass contained neural, intestinal and other incompletely developed structures from various parts of the body. Histopathology exam showed tissue from all the three germ layers including neural, gastrointestinal, renal, muscle, bone and cartilage tissue. The child was symptom free 3 years after surgery.

Figure 3: Thoraco-abdominal fetus in fetu mimicking a small fetus with head and body and par- tially formed axial skeleton


Case 4

A 3 months female infant presented with a classical sacrococcygeal teratoma (SCT). CT scan suggested a type I SCT with no deeper extensions and also calcifications in the mass. Excision of the mass was performed including coccygectomy. X-ray of the excised specimen showed partially formed pelvis, axial skeleton and also limb bones (Fig. 4). Histopathology showed muscle, bone, cartilage, skin appendages, and neural tissue. The child was symptoms free at 2 years follow-up.

Figure 4: Sacrococcygeal fetus in etu showing partially formed pelvic, limb bones and axial skeleton.



Fetus in fetu is a rare condition reported in about 1:500,000 live births [1]. The condition is twice as common in female as in males. In our study 3 patients were female and one male. The incidence is similar to SCT where the incidence of female to male is 3:1. FIF is a heterozygous mass containing tissue from various germ layers. Johann Fredrick Meckel first coined the terminology of fetus in fetu and suggested that it is a parasitic twin found in the abdomen of a sibling [4]. The confusion between mature cystic teratoma (MCT) and FIF still exists. Nicholson and Willis formulated the criteria for fetus in fetus diagnosis and described it as a diamniotic, monozygotic, monochorionic, twining with incorporation of one of the twins in the host [5]. Essentially they proposed that FIF is a monozygotic twin. Willis suggested that presence of an axial skeleton is the essential component of a teratomatous mass to label it as fetus in fetu. According to Willis the teratomas do not go through a stage of primitive streak therefore they do not have the ability to form an axial skeleton. Other however did not agree to this proposition and described FIF as a highly mature teratoma [6].

Some authors do not consider presence of axial skeleton as an essential component of fetus in fetu. They suggested that in the absence of a vertebral column, presence of a well-circumscribed, fluid-filled sac enclosing a solid mass and calcifications on sonography is sufficient for the diagnosis of fetus in fetu [7]. In our study we have only included those cases where an axial skeleton is clearly demonstrated as we see lots of patients with mature teratoma with cystic and solid masses with calcifications but they definitely do not classify as fetus in fetu.

Different studies have been done to establish the cause of FIF and most genetic analysis revealed that the FIF and the host have same genetic makeup [5-8]. This means that either FIF is tissue from the host or a monozygotic twining. In our study, out of four cases two patients had retroperitoneal FIF and can be explained on the basis of monozygotic twining theory; however, two patients had FIF in sacrococcygeal region which is a common site for SCT. This does enforce the fact that patients having mature teratoma can develop as FIF. This is an interesting finding as very few patients have been reported with FIF in sacrococcygeal area.

Histopathological exam of these masses is also interesting, as one of the presacral sacrococcygeal teratoma showed renal, neural, intestinal, cartilage, bone and muscle tissue and so was seen in the thoraco-abdominal FIF; however, one of the retroperitoneal FIF showed skin, hair follicles, sweat glands, glial tissue, respiratory mucosa and no intestinal tissue. The other FIF in the sacrococcygeal area showed muscle, bone, cartilage, skin appendages, and neural tissue. This does suggest that the tissue distribution may differ in different FIF irrespective of their origin. These cases may give us the information that FIF can well be a form of mature teratoma rather than monozygotic twining.

Lastly, even though FIF is a fascinating terminology and gives the impression that there is a baby incorporated in another baby, its management and that of a mature teratoma is essentially the same - complete excision. A permanent cure may be expected in case of FIF. Only one case of malignancy in FIF has been reported in the literature therefore the diagnosis does give an assurance for cure after resection [9]. The better outcome is because of the mature tissue in FIF. On the other hand the incidence of malignancy in retroperitoneal teratoma is up to 10% [10].


The differentiation between cystic mature teratoma and fetus in fetu may be difficult. The initial proposition that FIF is a parasitic twin may be true in cases of retroperitoneal masses however the presence of FIF in the sacrococygeal area supports the theory that it is a form of mature teratoma. The important fact is that FIF offers a cure after complete resection.




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