Thoracopagus: The most common type of conjoined twin, which along with omphalopagus, represents about 75 percent of cases. The two babies lie face to face and share a common sternum, diaphragm and upper abdominal wall. In cases of conjoined hearts at ventricular (pumping chamber) level, there are no known survivors.

Omphalopagus: The least-complicated connection, usually considered a subgroup of thoracopagus. The babies face one another and are joined at the anterior abdominal wall from xiphoid to umbilicus. The peritoneal cavity of one tends to be joined with the other, but upper intestinal tracts are usually separate. In the majority of cases, a bridge of liver connects the infants.

Pygopagus: Represents about 20 percent of cases, joined at the buttocks and perineum, facing away from each other. A significant length of the sacrum may be fused, and as a result, the twins often share the sacral spinal canal. A single lower rectum and anus is common, and often the lower genital tract and external genitalia are fused.

Ischiopagus: Represents less than 5 percent of cases, the connection occurs at a single bony pelvis. Four normal legs may be attached, but often two of the four are fused into one malformed limb. The intestinal tracts often join and empty into a single colon.

Craniopagus: The least common type of conjoined twins, accounting for 2 percent of cases, is represented by fusion of the skull. The twins often share large dural sinuses and vascular structures. Rarely, the brains are separated by bone and each brain has separate leptomeninges. In others, the brains are connected, or separated only by arachnoid, making separation extremely difficult and dependent upon a superior sagittal sinus for each brain.