Types of Conjoined Twins
Conjoined twins have fascinated people for centuries. They have been worshipped as gods and feared as monsters. They play a role in our myths and are marvelled at in circus sideshows. Once called "monstrosities," conjoined twins are increasingly accepted into our everyday lives as we grow to understand their unusual physical and emotional bonds and learn more about the science behind their development.
- are identical twins who develop with a single placenta from a single fertilized ovum.
- are always the same sex and race.
- are more often female than male, at a ratio of 3:1.
- occur as often as once in every 40,000 births but only once in every 200,000 live births.
- are more likely to occur in India or Africa than in China or the United States. may be caused by any number of factors, being influenced by genetic and environmental conditions.It is presently thought that these factors are responsible for the failure of twins to separate after the 13th day after fertilization. Conjoined twins can be artificially generated in amphibians by constricting the embryo so that two embryos form, one on each side of the constriction.
There are no documented cases of conjoined triplets or quadruplets.
Types of Conjoined Twins
Conjoined twins are usually classified by the point at which they are joined (the Greek word pagos, meaning "that which is fixed"). There have been as many as three dozen separate types identified in the last century. The following basic classifications can be combined to more closely define individual cases
1. Conjunction never involving heart or umbilicus.
- Craniopagus: Cranial union only, about 2% of all conjoined twins.
- Pygopagus: Posterior union of the rump, about 19% of all conjoined twins.
2. Conjunctions always involving the umbilicus.
- Thoracopagus: Anterior union of the upper half of the trunk. The most common form of conjoined twins (about 35%), it always involves sharing the heart.
- Cephalopagus: Anterior union of the upper half of the body with two faces on opposite sides of a conjoined head. Extremely rare. The heart is sometimes involved. A combination of types 3 and 4 is called cephalothoracopagus.
- Parapagus:(sometimes balled diprosopus): lateral union of the lower half, extending variable distances upward, about 5% of all conjoined twins. Heart sometimes involved.
- Ischopagus: Anterior union of the lower half of the body, about 6% of all conjoined twins. Heart not involved.
- Omphalopagus: Anterior union of the midtrunk, about 30% of conjoined twins.
3. Rare forms of conjoined twins, having different patterns.
- Parasitic twins: Asymmetrical conjoined twins, one twin being small, less formed, and dependent upon the other.
- Fetus in fetu: Situation in which an imperfect fetus is contained completely within the body of its sibling.