Monozygotic twins

These arise from a single fertilized oocyte and therefore have the same genetic information. In a third of the cases, the embryos become separated in the blastomere stage. In this case, each embryo has its own amnion and chorion. In two-thirds of the cases, separation occurs only in the stage of the blastocyst. These twins then have individual amnions and a common chorionic cavity. There are rare cases where the embryoblast divides in a later stage. The resulting twins possess a common amnion and chorion.
If the embryonic discs do not separate completely, the twins remain partially attached to each other (conjoined twins, Siamese twins). They are generally monozygotic and have a common amnion and chorion. The contact between both twins may occur at different regions of the body.