Embryology
During embryonic development, twins can form from the splitting of a single embryo (monozygotic)which forms identical twins or the twins can arise from separate oocytes in the same menstrual cycle (dizygotic) which forms fraternal twins. Although the later is more frequent, monozygotic is the reason conjoined twins can develop. In monozygotic twinning for conjoined twins such as ischiopagi, the twins form by the splitting of a bi-laminar embryonic disc after the formation of the inner cell masses. Thus, making the twins occupy the same amnion which can lead to a conjoining of the twins as a result of the twins not separating properly during the twinning process. Separation occurring between the seventh and thirteenth days should result in a monochorionic, monoamniotic identical twins sharing a yolk sac. If separation of the twins occur in the later stages of development prior to the appearance of the primitive streak and axial orientation, then it can be predicted that conjoined twins will develop. The origin of exactly what goes wrong to produce ischiopagus or any conjoined twin is a result by either incomplete fission or double overlapping inducing centers on the same germ disc. Various studies suggest that mechanical disturbances such as shaking of the blastomeres, exposure of the embryo to cold or insufficient oxygen during the early process of cleavage, grafting organizer onto gastrula or half a gastrula together, or constricting the blastula or early gastrula can cause the incomplete separation of monozygotic twins. However, studies have shown that these disturbances must happen at critical times in the pregnancy for the conjoined twins to develop.
Read more about this topic: Ischiopagi