A<unk>  (also known as an asymmetrical or unequal conjoined<unk>  is the result of the processes that produce vanishing<unk> s and conjoined<unk> s, and may represent a continuum between the two.<unk> s occur when a<unk>  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<unk> s, one ceases development during gestation and is vestigial to a mostly fully-formed, otherwise healthy individual<unk> . The undeveloped<unk>  is defined as<unk> , rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.
Parasitic twin
Fetus in fetu
Infant of diabetic mother
Dolly 
Anembryonic gestation
Retained products of conception
Breech birth
Instillation abortion