Parasitic twins are also called asymmetrical or unequal conjoined twins. They are said to be a type of conjoined twins since it is the same process that produces vanishing twins as well as conjoined twins. It occurs in one per one million births.
Parasitic twins are identical twins, this means that a single egg cell gets fertilized and splits in two after fertilization, thereby resulting in two separate fetuses. Regardless of this, if an egg fails to detach from the other completely, it can result in conjoined twins.
In some cases, one of the fetuses starts absorbing the other during the embryonic developmental stage. The fetus that gets partially absorbed by the other stops developing and hence becomes parasitic whereas the other keeps developing and becomes the dominant twin.
Hence, it is said that parasitic twins occur when a twin embryo begins developing in the utero but the pair does not fully separate and one embryo maintains a dominant development at the expense of it's twin.
There are many opinions, theories and postulations regarding the formation of parasitic twins but what exactly leads to it cannot be stated.
The twin that is undeveloped is termed parasitic as it is totally dependent on the body functions of the complete fetus. The independent or complete fetus is called the autositic twin.
The parasitic twin appears in the form of tissues or limbs on the autositic twin. It is either the partially developed twin attaches itself to the fully developed twin or one small body part gets duplicated. Practically, they are physically composed of a single properly functioning fetus who carries extra organs.
They are named based on the site of attachment of the organs to the autositic twin
– Epigastric : At or above the upper abdomen
– Omphalopagus : At the abdomen
– Craniopagus parasiticus : At the skull/cranium
– Rachipagus : At the back
– Pygomelia : Extra limbs attached at the buttocks and may then resemble a false tail
Unlike conjoined twins, they do not share organs. A pedicle of soft tosses containing large blood vessels connects the parasitic twin to the other twin.
Surgical removal of the undeveloped twin is the best way to save the autositic twin from the pain and burden of the other. It is also the best way to save the life of the autositic twin as carrying the undeveloped twin can cause huge medical complications.
Sometimes after surgery, little complications may come up in the body of the autositic twin which is why follow-up surgeries are advised.
Sources : Wikipedia, Flo.health