The “parasitic twin”, a developmental anomaly as rare as it is spectacular

When the twins share the same amniotic sac in their mother's womb, it happens in extremely rare cases that one of the two embryos involuntarily “swallows” the second. This developmental anomaly can only concern twins bathing in the same amniotic cavity, that is to say monozygotic twins or “identical twins”. This phenomenon drastically limits the development of the so-called “parasitic” twin without it disappearing, which gives rise to surprising malformations. What remains of the parasitic twin and can we live with it? Answers with Philippe Charlier, French medical examiner, anatomo-pathologist, archaeo-anthropologist and paleopathologist, author of Autopsy of historical hearts (Tallandier, 2023).

Called "ischiopagus", and more commonly "fetus in fetus", this anomaly is extremely rare and only affects one in 500,000 births. According to a study by Hong Kong Medical published in 2015, only 200 cases have been described in the scientific literature. worldwide. Historically, it is to the "father of modern surgery", Ambroise Paré, that we owe one of the first descriptions, represented on a 16th century engraving.

The fusion of these two entities takes place after fertilization from the moment the embryos undergo cellular differentiations. "It's a sort of phenomenon of competition where the marginal individual is 'swallowed up' by the one who becomes, so to speak, the main one", explains Philippe Charlier. The parasitic individual does not disappear completely but develops in vestigial form in the body of its twin. “As long as this portion of the organ is nourished and vascularized, it will grow, but it will be very special and surprising,” emphasizes Prof. Chalier.

Generally the remaining parts of the twin appear as growths which correspond to abnormally developed limbs. In some cases, however, these buds will develop inside the host individual, manifesting as swellings. They can form at the back of the mouth (the floor of the soft palate), at the level of the vertex of the head, of the neck (at the location of the thyroid), in the central part of the thorax (the mediastinum), at the level of the testicles in men or the ovaries in women.

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The causes of this phenomenon remain misunderstood, especially since all cases of "fetus in fetus" do not seem to be explained by a twin pregnancy. "Other hypotheses suggest that the vestigial forms, which take on the appearance of limbs with sometimes even hair, teeth or organs, may originate from a tumor that grows under the influence of very complex cellular and tissue processors. explains Philippe Charlier. “We then speak of teratoma”. Unlike the parasitic twin, the teratoma is a rare tumor that forms from particular embryonic stem cells, the germ cells that are capable of forming gametes. Some believe that the pathological processes of "fetus in fetus" and teratoma overlap in several aspects, which sometimes makes it difficult to distinguish between the two phenomena.

Removing the members that belong to the parasitic twin tends to shock the imagination because the question arises as to whether it has a conscience and therefore, if its integrity is being affected. “This is a question that has been debated a lot. At the time, in the absence of knowledge on the subject, the priests wondered if one should consider that it was about one or two births, if one should celebrate one or two baptisms and, at the time of the death of the main individual, organize a funeral for one or two people,” says Professor Charlier. Thanks to important scientific advances, we now have that when only members of the parasitic twin develop, it is not considered a proper individual. "With exceptions, he does not have a conscience for the simple and good reason... that he has no brain", underlines Professor Charlier. Its development indeed stopped well before the differentiation of certain stem cells into neurons.

Note that from the moment there is a part of a sensitive man, that is to say if we observe two distinct cerebral developments, we speak of Siamese twins and not of parasitic twins.

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Thanks to the examinations carried out during pregnancy (3D ultrasound or even fetal MRI), the parasitic twin can be detected very early. However, most of the time, the “fetus in fetus” is diagnosed at birth or during childhood before the age of 18, without realizing it. This is all the more true if the fetus ends up inside its twin. It is therefore possible to live relatively long with a parasitic twin. However, this type of malformation can also be very disabling. This is why, as soon as the diagnosis is made, doctors consider surgery to remove the remaining parts of the parasitic twin.

It depends on how it developed. In all cases, these are heavy surgical operations which can prove to be very complicated anatomically. “For questions of anastomoses, that is to say the communication established between the two fetuses, it is sometimes very difficult or even impossible to remove the parasitic twin without endangering its twin. For example, let's imagine that two cores are partially merged: in this case, few options are possible,” recalls Philippe Charlier.