How twins are joined
Conjoined twins are usually classified according to where they’re joined, and there are many ways that conjoined twins may be connected. Some of the more common ways include:
- Joined at the chest. One of the most common of conjoined twins, thoracopagus twins are joined at the chest. They often have a shared heart and may also share one liver and upper intestine.
- Joined near the bellybutton. Omphalopagus twins are joined near the bellybutton. Many omphalopagus twins share the liver, and some share the lower part of the small intestine (ileum) and colon. They generally do not, however, share a heart.
- Joined at the base of the spine. Pygopagus twins are joined at the base of the spine and commonly face away from one another. Some pygopagus twins share the lower gastrointestinal tract, and a few share the genital and urinary organs.
- Joined at the pelvis. Ischiopagus twins are joined at the pelvis. Many ischiopagus twins share the lower gastrointestinal tract, as well as the liver and genital and urinary tract organs. Each twin may have two legs or in some cases, one pair of legs and even a fused leg, though that’s uncommon.
- Joined at the head. Craniopagus twins are joined at the head. Craniopagus twins share a portion of the skull, and possibly brain tissue. This sharing may involve the cerebral cortex — the part of the brain that plays a central role in memory, language and perception.
In rare cases, twins may be asymmetrically conjoined, with one twin smaller and less fully formed than the other (parasitic twins).