These are twins that develop from the same ovum. They are always of the same sex and have the same arrangement of genes and chromosomes. They are usually alike in physical and mental character.
Monozygous twins may share the same placenta, chorion and amnion. The amount of commonality of these organs is dependent on the stage at which division occurred; those which arise from early division are more likely to each have a chorion and an amnion, whilst those from later divisions, are more likely to share a chorion but have separate amnions.
Monoamniotic twins are rare and associated with increased fetal loss from cord entanglement. They usually have distinct umbilical cords but anastomoses in the placentae may result in communication of their fetal circulations. Twin to twin transfusion may result in unequal blood flow between the two twins, causing one fetus to become plethoric and the other undernourished and anaemic. The latter may eventually die and if this occurs in early pregnancy, may be undetected until delivery when it is observed compressed flat on the membranes - fetus papyraceous.
Conjoined twins may arise when the process of division of a single germinal area is incomplete.
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