Conjoined twins. Skeleton of conjoined (siamese) twins showing that each twin had its own head, neck and trachea (windpipe), but shared the rest of the body. The medical name for a two-headed infant is dicephalus dibrachius. Most of these are stillborn or die soon after birth, although a few have survived beyond that. As with identical twins, conjoined twins result from the fertilisation of a single egg cell, but in this case the spitting of the embryo into two identical ones is incomplete.