Twin Sisters

Regarding spontaneous or natural monozygotic twinning, a recent theory posits that identical twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.
Identical twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.
• Incidence
Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.