What is a parasitic twin?
A vestigial parasitic twin is actually a type of conjoined twin, which isn’t fully formed and relies entirely on the autositic or complete twin’s body. Note that there are two main versions of conjoined twins:
- Equal or symmetrical
Two equally well-developed fetuses
- Unequal or asymmetrical
Either a partially developed twin attaches to a fully developed twin, or one small body part gets duplicated
When one conjoined twin stops growing during pregnancy, they become rudimentary to the dominant or autositic twin, resulting in a parasitic twin. The parasitic twin is either nonfunctional, incompletely formed, or totally dependent on their sibling. Consequently, vestigial parasitic twins are physically composed of a single, properly functioning fetus who carries extra organs or appendages.
Parasitic conjoined twins don’t typically share their organs and their fetal parts are grossly recognizable. A pedicle of soft tissues containing large blood vessels connects the parasitic twin to the other twin. Often times, a parasitic twin will be named based on the site of attachment:
- Cephalopagus
At the back of the head
- Epigastric
At or above the upper abdomen
- Omphalopagus
At the abdomen
- Craniopagus parasiticus
At the skull or cranium
- Ischiopagus
At the pelvis
- Parapagus
At the abdomen and pelvis
- Pyopagus
At the lower spine
- Rachipagus
At the back
- Thoracopagus
At the upper chest
- Pygomelia
Extra limbs attach at the buttocks and may resemble a false tail
Without treatment, the dominant twin is vulnerable to serious medical complications from supporting the vestigial twin.