Placental teratomas thought to arisefrom germ cells migrating into extraplacental membranes. Historically, placental teratomas were differentiated from acardiac amorphoustwins by presence of multiple tissuetypes, but lack of axial organization andumbilical cord. New evidence - previously documented placental teratomas may insteadrepresent acardiac amorphous twin given similar pattern of short tandem repeat (STR) polymorphic loci between placental masses (thought to be teratoma) and adjacent normalplacental tissue.