Have you ever wondered what your life would be like if you had an identical twin? Is someone else with the exact same DNA as you? Would you be best friends, attached at the hip, or would you embark on divergent life paths? What if I told you it is possible that you have a twin that you didn’t know about, and that they might be inside of you?

Unfortunately, in the case of foetus-in-foetu, you’ll never be able to meet your twin as it describes a phenomenon when one twin in a conjoined pair does not make it past the foetal stage. It presents as a mass on the dominant twins’ body, often in the abdomen and noticed during infancy. It is so rare that there isn’t a strict list of symptoms and can be confused for a tumour.

Causes

Twins can come from one sperm fertilising an egg then the zygote (pre-embryo) splits in two (identical twins), or two sperm fertilising two eggs (fraternal twins). Conjoined twins describe twins born physically attached or sharing body parts. They are believed to arise either when the zygote split occurs too late resulting in a partial separation, or after separation they fuse together due to their close proximity in the womb. This can only occur with identical twins.

When one twin is less developed than the other, called asymmetrically conjoined twins, it is thought to be due to the zygote split being unbalanced or a complication that prevents the twin's development. Parasitic twins occur when the underdeveloped twin has some structures and organs, but a lack of a functioning heart, and is therefore completely dependent on the dominant twin. It is extremely rare. About 1 in every million births is parasitic twins.¹

Foetus-in-foetu is a parasitic twin inside the body, having never developed past the foetal stage. There is another theory that the fertilised egg never fully separated, or that vascular (blood vessel) problems cause the tissue of the parasitic foetus to become dependent on their twin and atrophy (wasting away) occurs due to the lack of blood supply. It predominantly occurs in the upper retroperitoneum, the area of the abdomen where the liver, stomach, spleen, and small intestine reside.²

In many cases, healthcare professionals believe that the foetus in foetus-in-foetu is not a foetus at all. The foetus is a teratoma, a germ cell tumour. Germ cells are unspecialised, meaning they can eventually develop into all kinds of cells, including egg and sperm cells. When the cells mutate into a tumour, the tumour itself can develop blood cells, nerve cells, muscle cells, hair cells, bone cells and many more. These tumours are often found around reproductive organs like the ovaries and the testes, but also the tailbone. One in ten teratomas are cancerous.³

Foetus-in-foetu and a teratoma are differentiated usually from the presence of a vertebral column and appropriately situated bones and organs, indicating that it may have gone through the early stage of foetal development.³ Although it would be useful to differentiate between the two due to the cancerous potential of teratomas, many research papers consider foetus-in-foetu and mature teratomas so similar that they are considered the same thing.

Symptoms

Because this condition is so rare, the exact symptoms are unclear. It can be asymptomatic. In some reported cases, the subject experiences vomiting, abnormality in appearance and feel around the location, and pain. The mass may get larger unevenly, be full of fluid, and cause abdominal distention. It will not affect pregnancy or childbirth. There is no particular bodily location where these foetuses appear, and there is no particular age at which the symptoms manifest, although many reported cases occurred in the abdomen during early infancy.

The mass/foetus would be identified as an underdeveloped twin due to the presence of body parts, like a head, spinal cord, or limbs.

A teratoma can produce pain in its location, bleeding, swelling, elevated levels of the BhCG and AFP hormone, or a visible mass. If the teratoma originated in the tailbone it may cause constipation, painful urination, or weakness in the legs.

Diagnosis

With the advancement of imaging, it is easier to diagnose foetus-in-foetu than ever before. It is possible that it can be diagnosed in a pre-natal ultrasound if it is particularly large. In most cases, the mass is noticed and dealt with in early infancy.

Some imaging options include an X-ray, an MRI, a CT scan, or a PET scan.  A healthcare professional may begin with a physical examination. They may also perform an investigation of the tumour and your blood work. It is important to identify if the mass is a foetus-in-foetu or a teratoma, due to the cancerous potential of teratomas.

X-Ray

X-rays are a very quick and painless imaging technique that would allow a healthcare professional to see your bones. It may be required that you stop taking certain medications and eating and drinking if they use a contrast agent. This can show the skeleton of the mass.

MRI (Magnetic resonance imaging)

MRIs consist of strong magnetic fields and radio waves that produce a detailed image of the inside of your body. These can be very loud, so you’ll be given earplugs or headphones to wear. They can last up to 90 minutes and are painless. Due to the strong magnetic field, those with a pacemaker or an artificial joint cannot have an MRI. It’s helpful for a few reasons, but this would allow soft tissue of the mass and surrounding areas to be viewed in high resolution, like the spine and pelvic anatomy.²

CT Scan (Computer tomography)

A CT scan lying in a rotating X-ray tube for approximately 15 minutes. You are given a dye to show contrast to the body part of interest. It may be drunk, injected, or taken through the bottom. Other than the injected contrast dye, which may lead to feeling hot, tasting metal, and feeling like you’re weeing, it is a painless procedure. It can display a 3D image of the skeleton of the mass (the vertebrae and limbs) and organs near it.

PET Scan (Positron emission tomography)

A PET scan requires the patient to lie down in a cylindrical positron detector. A substance called fluorodeoxyglucose that emits positrons is injected into your arm. It is a harmless substance like naturally occurring glucose (sugar). Cancerous cells use glucose quicker than non-cancerous cells, which can be shown in the images produced. You won’t be able to eat anything 6 hours before the scan. It can take between 30 and 60 minutes.

Blood work

Blood work would identify tumour markers that are produced by cancerous cells.

Biopsy

The healthcare professional may take a part of the tumour and inspect it under a microscope. This is the only way to know for sure whether it is cancerous or not. Depending on where the tumour is, it may involve a needle, cutting the skin, or inserting an endoscopy.

Treatment

The foetus does not necessarily need treatment, however, it may continue to grow and cause further complications. In this case, surgery is most likely required to remove the tumour.

If the tumour is found to be cancerous, some therapies like chemotherapy or radiation therapy will be required. More imaging of the foetus will most likely be required pre-surgery.

If surgical intervention is required it will be unique to the patient. They will vary greatly in risk. Removal of the mass without any damage to the surrounding structures is very common, although there is potential for complications, especially if the foetus is attached to the spinal cord or receives its blood supply from the main artery.⁴

FAQs

How common is foetus-in-foetu?

It is extremely rare. There are less than 100 cases that have been reported in medical literature.⁵

How do I know if I have a foetus-in-foetu?

Severe cases of foetus-in-foetu are often noticed and dealt with in early infancy. If you believe you have a foetus-in-foetu, it is most likely small and harmless. Discovering any abnormalities, especially lumps, on your body should be followed up with a healthcare professional as soon as possible to identify any cancerous tumours and deal with them early.

Are foetus-in-foetu and parasitic twins the same thing?

Foetus-in-foetu is an internal parasitic twin that never developed past the foetal stage. 

Are foetus-in-foetu and teratomas the same thing?

A teratoma is a tumour that has developed body parts similar to that of a foetus. A teratoma is so similar to a foetus-in-foetu that some recorded cases are ambiguous as to which condition it is.

Is foetus-in-foetu pregnant with your own twin?

No. Foetus-in-foetu can happen to anyone of any sex and does not have a particular bodily location in which it resides.

Summary

• A foetus-in-foetu is an undeveloped twin that tends to appear as a mass in the abdomen
• It is due to a conjoined twin that was unable to develop. It is theorised to be due to a lack of blood supply
• It is often diagnosed during early infancy
• It can be asymptomatic, but it can also be painful and occasionally fatal
• It is very similar to a teratoma, a tumour made of cells that can develop into various body parts that can often resemble a foetus
• It may require surgical intervention. These surgeries are often unique and vary in risk depending on the location of the foetus and whether it’s connected to the main artery