Parasitic twin

A parasitic twin, also called an asymmetrical twin or unequal conjoined twin, is a rare congenital condition that occurs when a twin embryo begins developing inside the womb but the pair of twins never fully separates, so that one embryo continues developing normally while the other stops developing and becomes dependent on the body of its sibling. The condition is thought to arise from the same underlying processes that also produce vanishing twins and fully conjoined twins, and researchers view it as sitting on a continuum between those two outcomes. The underdeveloped twin, called the parasitic twin, is incompletely formed and relies on the body functions of the fully formed twin, known as the autosite. Together the autosite and its parasitic twin are referred to as heteropagus twins. Because the parasitic twin typically has an underdeveloped or absent brain, it is unable to speak, though in some cases it retains limited ability to move a mouth or hand; no case of a parasitic twin able to speak has ever been recorded.