Editing Conjoined twins (section)

== Causes ==
There are two hypotheses about the development of conjoined twins.
# 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.<ref>{{cite book |title=DC Dutta's textbook of obstetrics |isbn=9789351527237 |page=233 |edition=Eighth|last1=Konar |first1=Hiralal |date=May 10, 2015 |publisher=JP Medical }}</ref>
# fusion of two fertilized eggs occurs early in development.

Partial splitting of the [[primitive node]] and [[Primitive streak|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 [[Homeobox protein goosecoid |goosecoid]], may also result in conjoined twins.<ref>{{cite book |title=Langman's medical embryology |isbn=9781496383907 |page=124 |edition= 14th |last1=Sadler |first1=Thomas W. |date=October 29, 2018 |publisher=Wolters Kluwer }}</ref> 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.<ref>{{cite book |title=Langman's medical embryology |isbn=9781496383907 |page=63 |edition= 14th |last1=Sadler |first1=Thomas W. |date=October 29, 2018 |publisher=Wolters Kluwer }}</ref>