Siamese twinning occurs when the inner cell mass derived from a single zygote divides 12 days or more after fertilization.
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Conjoined twins are typically classified by the point at which their bodies are joined. The common forms are:
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1. Thoraco-omphalopagus: Two bodies fused from the upper chest to the lower chest. These twins usually share a heart, and may also share the liver or part of the digestive system.
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2. Thoracopagus: Two bodies fused from the upper thorax to lower belly. The heart is always involved in these cases. As of 2015, separation of a genuinely shared heart has not offered survival to two twins; a designated twin may survive if allotted the heart, sacrificing the other twin.
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3. Omphalopagus: Two bodies fused at the lower abdomen. Unlike thoracopagus, the heart is never involved in these cases; however, the twins often share a liver, digestive system, diaphragm and other organs.
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4. Parasitic twins: Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin for survival.
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5. Craniopagus: Fused skulls, but separate bodies. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.
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