Identical twins happen when a single sperm (male cell), fertilizes a single egg (female cell) that then splits in two. Each one goes on to grow into a baby, but both cells share exactly the same genetic instructions (DNA) . DOUBLE (OR TRIPLE) TROUBLE! The number of births in which twins or triplets are likely to occur naturally: 1: Identical twins 4 in 1000 births 2: Non-identical (fraternal) twins 15 in 1000 births 3: Identical triplets 1 in 100000 births 4: Non-identical triplets 1 in 8000 births