Terata Catadidyma refers to twins joined in the lower portion of the body, or may appear to be two bodies on top and one body on the bottom.
Pygopagus - back-to-back, joined at the rump (about 19 percent)
Ischiopagus - joined sacrum to sacrum (about 6 percent)
Dicephalus - one body with two separate heads
Diprosopus - single body and head, but bearing two faces
dicephalic parapagus have one trunk and two heads, and may have
two (dibrachius), three (tribrachius), or four (tetrabrachius) arms.
Duplicata incompleta, (Parapagus) dicephalus dibrachius dipus (PDDD) type has two heads on one body
"dipygus dibrachius tetrapus",
"'posterior dichotomy,' subvariety schizorachis"
Terata Anadidyma refers to twins with one single upper body with a double lower half or twins who are connected by a single body part.
Cephalopagus - connected at the head (about two percent)
Syncephalus - connected in the facial region
Cephalothoracopagus - connected in the facial region and at the thorax
Dipygus - one upper body with two lower bodies (including the abdomen, pelvis and legs)
Caudal Duplication Anomalies
Terata Anacatadidyma refers to twins who are joined somewhere along the midsection of the body.
Thoracopagus - joined at the chest; may share a single heart or have some cardiac connection; some abdominal organs may be malformed
Omphalopagus - joined at the chest (about 33 percent)
Rachipagus - back-to-back, joined along the spine above the sacrum