Parasitic twin

Medical condition

A parasitic twin, also known as an asymmetrical twin or unequal conjoined twin, occurs when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of the other. It results from the same processes that also produces vanishing twins and conjoined twins, and may represent a continuum between the two. In parasitic twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is termed as parasitic, because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite.

This keyword could refer to multiple things. Here are some suggestions: