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:
At the back of the head
At or above the upper abdomen
At the abdomen
- Craniopagus parasiticus
At the skull or cranium
At the pelvis
At the abdomen and pelvis
At the lower spine
At the back
At the upper chest
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.