A parasitic twin, also known as an asymmetrical twin or unequal conjoined twin, occurs when a

twin Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of Twin Last Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two ...

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of the other. It results from the same processes that also produces '' vanishing twins'' and ''

conjoined twins Conjoined twins, popularly referred to as Siamese twins, are twins joined '' in utero''. It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births to one in 200,000 births, with a somewhat higher incidence in south ...

'', and may represent a continuum between the two. In parasitic 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 pregn ...

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 termed as ''

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

'', because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite. The autosite, together with the parasite, are collectively referred to as heteropagus twins.

Variants

TRAP sequence

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 twi ...

, or TRAP 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 (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

. The parasitic twin, little more than a

torso The torso or trunk is an anatomical terminology, anatomical term for the central part, or the core (anatomy), core, of the body (biology), body of many animals (including human beings), from which the head, neck, limb (anatomy), limbs, tail an ...

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 TRAP 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 TRAP/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.

Others

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 is a general term for a parasitic head attached to the head of a more fully developed fetus or infant. Fetus ''in fetu'' sometimes is interpreted as a special case of parasitic twin, but may be a distinct entity.

Gallery

Human

File:Gould Pyle 54.jpg, German man, born 1516 File:Histoires prodigieuses; Homme monstueux... WMS 136 Wellcome L0025563.jpg, 40-year-old man seen in

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...

, 1530 File:Gould Pyle 55.jpg, Lazarus and Joannes Baptista Colloredo, born 1617 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.jp
Blanche Dumas
of France, born 1860 File:Gould Pyle 56.jpg, Louise L., born 1869 File:B. C. Hirst & G. A. Piersol, Human monstrosities Wellcome L0027956.jpg, Joao Baptista dos Santos and Louise L. (), 1893 File:Gould Pyle 57.jpg, Laloo, 1897

Animals

File:Les écarts de la nature page12.jpg,

Chicken The chicken (''Gallus gallus domesticus'') is a domesticated subspecies of the red junglefowl (''Gallus gallus''), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is now one of the most common and w ...

with two 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 small heads, relatively short necks and slender bills that in some species feature fleshy ceres. ...

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

Cat The cat (''Felis catus''), also referred to as the domestic cat or house cat, is a small domesticated carnivorous mammal. It is the only domesticated species of the family Felidae. Advances in archaeology and genetics have shown that the ...

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

Sheep Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal 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 d ...

with two extra front limbs

Variants

TRAP sequence

Others

Gallery

Human

Animals

See also

References

Further reading