Twin Sisters

Identical or monozygotic (MZ) twins occur when a single egg is fertilized to form one zygote (hence, "monozygotic") which then divides into two separate embryos.
There are an estimated 11 million sets of identical twins and triplets in the world today.
• Mechanism
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.