Parasitic twin
Encyclopedia
A parasitic twin is the result of the processes that produce vanishing twin
s and conjoined twins
, 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 the other. Unlike conjoined twins, one ceases development during gestation
and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, 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.
Vanishing twin
A vanishing twin, also known as fetal resorption, is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed by the mother or twin....
s and conjoined twins
Conjoined twins
Conjoined twins are identical twins whose bodies are joined in utero. A rare phenomenon, the occurrence is estimated to range from 1 in 50,000 births to 1 in 100,000 births, with a somewhat higher incidence in Southwest Asia and Africa. Approximately half are stillborn, and a smaller fraction of...
, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero
In utero
In utero is a Latin term literally meaning "in the womb". In biology, the phrase describes the state of an embryo or fetus. In legal contexts, the phrase is used to refer to unborn children. Under common law, unborn children are still considered to exist for property transfer purposes.-See also:*...
, but the pair does not fully separate, and one embryo maintains dominant development at the expense of the other. Unlike conjoined twins, one ceases development during gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, 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 occipitalOccipital lobeThe occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
region or parietalis if joined in the parietalParietal lobeThe parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...
region. - Craniopagus parasiticusCraniopagus parasiticusCraniopagus parasiticus is a medical condition in which a parasitic twin head with an undeveloped body is attached to the head of a developed twin.-Cases:...
is a general term for a parasitic head attached to the head of a more fully developed fetus or infant. - Fetus in fetuFetus in fetuFetus in fetu is a developmental abnormality in which a mass of tissue resembling a fetus forms inside the body. There are two theories of origin concerning "fetus in fetu". One theory is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other theory is that the...
sometimes is interpreted as a special case of parasitic twin, but may be a triplet pregnancy complicated by acardius acormus. Fetal Diagnosis and Therapy 2009;26(1):45-9.