Foetus-in-Fetu (FIF)

by Isobella P-W

Foetus-in-fetu is a very rare disorder, occurring approximately once every 500,000 births. It presents itself as a mass with no definite form of a foetus (a malformed foetus) enclosed in the body of its twin. Therefore, an FIF twin is usually considered to be a parasitic foetus. It happens in identical or ‘monozygotic’ twins which are twins conceived from one egg and one sperm. The FIF is attached to the host twin via a body stalk containing a single main feeding artery and vein, these represent umbilical vessels.

However, the aetiology (the cause, set of causes or manner of causation of a disease or condition) of FIF is controversial, with two theories being proposed.

One of which is the theory of the ‘parasitic twin’. This is where a parasitic foetus is formed inside the body of its host twin. This foetus is usually malformed and usually dies before birth.

The second theory is of a highly differentiated form of teratoma. This is where there is a tumour composed of tissues not normally present at the site. As a result of this a smaller cell mass may remain within a maturing sister embryo. However, it is important to note that although this can look a lot like the malformed foetus we see in foetus-in-fetu this tumour does not have an amniotic sac or a placenta, therefore it is not able to grow and develop into a foetus. Some researchers believe that FIF represents one spectrum of a malignant tumour.

Foetus-in-fetu can go undiagnosed. Some of the symptoms of this disorder are abdominal distension, palpable mass, vomiting, poor feeding, jaundice (a yellow discolouration of the body tissue resulting from the accumulation of excess bilirubin) and/or dyspnea (shortness of breath). This disorder is characterised by the presence of vertebrae and well-developed organs in a fetiform mass. There have been less than 200 cases of FIF reported in medical literature. Of those 18, only 5 of them were prenatally detected, therefore, as previously mentioned, it is likely to go undiagnosed.

Antenatal (before birth occurs) diagnosis can be done by an ultrasound, CT and MRI. This is usually detected in infancy, with most cases being discovered before the age of 18 months. They are also usually detected in male gender, as it has a 2:1 male predominance. Additionally, the prognosis (the likely cause of a medical condition) is good if operated early.

Sources

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10230229/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3214284/

https://casereports.bmj.com/content/12/8/e230835

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