Parasitic Twins
Parasitic twins are a type of conjoined twin and occurs when one twin ceases development during gestation a becomes fully reliant on the fully formed dominant twin, also known as the autositic twin. The underdeveloped twin can be partially formed, not functional, or is wholly dependent on the autositic twin. Most researchers think they are the result of mechanisms similar to those that produce vanishing twin syndrome and conjoined twins. There are many different types of parasitic twins, but the most common is vestigial twins which is when one has extra limbs or organs which are generally harmless to the autositic twin. Parasitic twins come from incomplete twinning where instead of one egg separating Into two eggs to make two babies, the one egg does not fully separate.