With identical twins, one egg (zygote) from the mother is
fertilized by one sperm from the father, and then very early in
development the embryo splits and two fetuses grow. Spontaneous
division of the zygote into two embryos is not considered to be a
hereditary trait, but rather a spontaneous or random event.
If the zygote splits very early (in the first 2 days after
fertilization) they may develop separate placentas (chorion) and
separate sacs (amnion). These are called dichorionic, diamniotic
(or 'di/di') twins. While all fraternal twins are 'di/di', this
occurs 20 - 30% of the time in identical twins.
Most of the time in identical twins the zygote will split after
2 days, resulting in a shared placenta, but two separate sacs.
These are called monochorionic, diamniotic ('mono/di') twins.These
twins are very similar genetically, and share a single
afterbirth.
Very occasionally, twins will also share the same sac (fluid
cavity). In about 1% of identical twins the splitting occurs late
enough to result in both a shared placenta and a shared sac. These
are called monochorionic, monoamniotic ('mono/mono') twins.
