A parasitic twin, also known as an asymmetrical or unequal conjoined twin, is the result of the processes that also produce

vanishing twin A vanishing twin, also known as twin resorption, is a fetus in a multigestation pregnancy that dies '' in utero ''and is then partially or completely reabsorbed. In some instances, the dead twin is compressed into a flattened, parchment-like st ...

s and

conjoined twins Conjoined twins – sometimes popularly referred to as Siamese twins – are twins joined '' in utero''. A very rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence ...

, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin. Unlike conjoined twins, one ceases development during

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during preg ...

and is

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite.

Variants

* Conjoined parasitic twins joined at the head are described as craniopagus or cephalopagus, and occipitalis if joined in the occipital region or parietalis if joined in the parietal region. * ''

Craniopagus parasiticus Craniopagus parasiticus is an extremely rare type of parasitic twinning occurring in about 2 to 3 of 5,000,000 births. In craniopagus parasiticus, a parasitic twin head with an undeveloped body is attached to the head of a developed twin. Fewer t ...

'' is a general term for a parasitic head attached to the head of a more fully developed fetus or infant. * ''

Fetus in fetu ''Fetus in fetu'' (or ''foetus in foetu'') is a rare developmental abnormality in which a mass of tissue resembling a fetus forms inside the body of its twin. An early example of the phenomenon was described in 1808 by George William Young. T ...

'' sometimes is interpreted as a special case of parasitic twin, but may be a distinct entity. * The

twin reversed arterial perfusion Twin reversed arterial perfusion sequence, also called TRAP sequence, TRAPS, or acardiac twinning, is a rare complication of monochorionic twin pregnancies. It is a severe variant of twin-to-twin transfusion syndrome (TTTS). In addition to the ...

, or T.R.A.P. sequence, results in an 'acardiac twin', a parasitic twin that fails to develop a head, arms and a

heart The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon diox ...

. The parasitic twin, little more than a

torso The torso or trunk is an anatomical term for the central part, or the core, of the body of many animals (including humans), from which the head, neck The neck is the part of the body on many vertebrates that connects the head with the tors ...

with or without legs, receives its blood supply from the host twin by means of an umbilical cord-like structure, much like a ''fetus in fetu'', except the acardiac twin is outside the autosite's body. The blood received by the parasitic twin has already been used by the normal fetus, and as such is already de-oxygenated, leaving little developmental nutrients for the acardiac twin. Because it is pumping blood for both itself and its acardiac twin, this causes extreme stress on the normal fetus' heart. Many T.R.A.P. pregnancies result in heart failure for the healthy twin. This twinning condition usually occurs very early in pregnancy. A rare variant of the acardiac fetus is the ''acardius acormus'' where the head is well developed but the heart and the rest of the body are rudimentary. While it is thought that the classical T.R.A.P./Acardius sequence is due to a retrograde flow from the umbilical arteries of the pump twin to the iliac arteries of the acardiac twin resulting in preferential caudal perfusion, ''acardius acormus'' is thought to be a result of an early embryopathy.Abi Nader Khalil, Whitten Sara Melissa, Filippi Elisa, Scott Rose-Mary, Jauniaux Eric. Dichorionic triamniotic triplet pregnancy complicated by acardius acormus. Fetal Diagnosis and Therapy 2009;26(1):45-9.

Images of parasitic twins

Human

File:Histoires prodigieuses; Homme monstueux... WMS 136 Wellcome L0025563.jpg, 40-year-old man seen in

Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. ...

, 1530 File:Les écarts de la nature page28.jpg, 30-year-old Neapolitan man, seen in 1742 File:An Indian man with a headless body attached to his trunk. En Wellcome V0007399.jpg, Indian man, 1787 File:Agan (BM 1982,U.2878).jpg, Agan, a Chinese man, 1833 File:Gustav Evrard.jpg, 9-year-old Gustav Evrard of Paris, 1839 File:Dumas.3.jpg, Blanche Dumas of France, born 1860 File:B. C. Hirst & G. A. Piersol, Human monstrosities Wellcome L0027956.jpg, Joao Baptista dos Santos and Louise L. ("la femme de quatre jambes"), 1893

Animals

File:Les écarts de la nature page12.jpg,

Chicken The chicken (''Gallus gallus domesticus'') is a domestication, domesticated junglefowl species, with attributes of wild species such as the grey junglefowl, grey and the Ceylon junglefowl that are originally from Southeastern Asia. Rooster ...

with 2 extra lower limbs File:Les écarts de la nature page30.jpg,

Pigeon Columbidae () is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with short necks and short slender bills that in some species feature fleshy ceres. They primarily ...

with 2 extra lower limbs File:Les écarts de la nature page44.jpg,

Cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...

with lower body duplication File:Les écarts de la nature page45.jpg,

Sheep Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to domesticated sh ...

with 2 extra front limbs

Variants

Images of parasitic twins

Human

Animals

See also

References

Further reading