Conjoined twins are usually classified by the point at which they are joined (the Greek word pagos, meaning "that which is fixed.") Here are some of the main types:

Head-to-head, about 2% of all conjoined twins.

Pelvis-to-pelvis, about 19% of all conjoined twins.

Lateral union of the trunk, about 5% of all conjoined twins. Heart sometimes involved.

Fusion of head and thorax.

Chest-to-chest, the most common form of conjoined twins (about 35%).

The heart is usually shared and there are associated vascular abnormalities, complicating surgical separation

Adapted from: Carlson, B.M. (1994) Human embryology and developmental biology. St. Louis: Mosby-Year Book, Inc., p 43.