parasitic twin

Parasitic twin.

A parasitic twin is the result of the processes that produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur 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. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined asparasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.