The first is that a single fertilized egg does not fully split during the process of forming identical twins. If the zygote division occurs after two weeks of the development of the embryonic disc, it results in the formation of conjoined twins. The second theory is that a fusion of two fertilized eggs occurs earlier in development. Partial splitting of the primitive node and streak may result in the formation of conjoined twins. These twins are classified according to the nature and degree of their union. Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. The type of twins formed depends on when and to what extent abnormalities of the node and streak occurred. Misexpression of genes, such as Goosecoid, may also result in conjoined twins. Goosecoid activates inhibitors of BMP4 and contributes to regulation of head development. Over- or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of conjoined twins. Types
Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:

Thoraco-omphalopagus (28% of cases): 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. Thoracopagus (18.5%): Two bodies fused from the upper chest to lower belly. The heart is always involved in these cases.
Inflammatory diseases of the blood vessels, like giant-cell arteritis, polyarteritis nodosa, Churg–Strauss syndrome, granulomatosis with polyangiitis, and rheumatoid arthritis can cause arteritic AIONs (AAION). The vast majority of AIONs are nonarteritic AIONs (NAION). The most common acute optic neuropathy in patients over 50 years of age, NAION has an annual incidence of 2.3-10.2/100,000. NAION presents as a painless loss of vision, often when awakening, that occurs over hours to days. Most patients lose the lower half of their visual field (an inferior altitudinal loss), though superior altitudinal loss is also common. The pathophysiology of NAION is unknown, but it is related to poor circulation in the optic nerve head. NAION is often associated with diabetes mellitus, elevated intraocular pressure (acute glaucoma, eye surgery), high cholesterol, hypercoagulable states, a drop in blood pressure (bleeding, cardiac arrest, peri-operative esp. cardiac and spine procedures), and sleep apnea. Rarely, amiodarone, interferon-alpha, and erectile dysfunction drugs have been associated with this disease. Posterior ischemic optic neuropathy is a syndrome of sudden visual loss with optic neuropathy without initial disc swelling with subsequent development of optic atrophy. This can occur in patients who are predisposed to AAION and NAION as described above as well as those who had cardiac and spine surgery or serious episodes of hypotension. Radiation optic neuropathy (RON) is also thought to be due to ischemia of the optic nerve that occurs 3 months to 8 or more years after radiation therapy to the brain and orbit.