Monozygotic or identical twins are those that originate from a single ovary_ and a single sperm and therefore share the same genetic load.
What happens in these cases is that the embryo splits in two, and depending on the moment in which this happens, different configurations can be expected in the development of the placenta.
When the embryo is cleaved within the first four days after fertilization it results in a bicorial-biamnotic pregnancy, that means, two sacs and two independent chorions will be visible (exactly the same as in the case of dizygotic twins from two different embryos).
