The origin of monozygotic twinning.

The incidence of monozygotic twinning appears to be increasing within the field of assisted human reproduction. Many theories have been put forward as to how and when this occurs. Whatever the cause, the normal events of embryo development, which necessarily involve axis formation, patterning and polarization, need to be adhered to in order to obtain a viable offspring. This paper describes the course of development in terms of axis formation and polarity and offers suggestions as to how either a disruption of this or duplication events in the course of the formation of these parameters could prevent or contribute to a twinning event. The likelihood of twinning occurring at any point is discussed in terms of the establishment of polarity and axes.