There are two theories about the development of<unk> . 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<unk> . 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<unk> . 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<unk> . 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<unk> .
Conjoined twins
Locked twins
Monochorionic twins
Monoamniotic twins
Vanishing twin
Parasitic twin
Craniopagus twins
Vestigial twin