Conjoined twins are twins whose skin and some internal organs are joined together. About 35% of conjoined twins survive for only a day, 40-60% of conjoined twins are stillborn, and only about 5-25% of conjoined twins survive into adulthood. Although there are many different types of conjoined twins, the most common type is thoracic conjoined twins, which are joined at the upper body. In some cases of conjoined twins, they will have only one heart. Less common are cephalic conjoined twins, in which the twins are joined at the head or skull.

Conjoined twins are caused by the embryos only partially splitting, rather than forming two separate individuals. Although the twins develop together, they will be physically connected to each other - most often in the chest, abdomen or pelvis - and will share one or more internal organs.