In the United States, it is estimated that for every 1,000 children born, 33.3 of them will be twins; 22.8 of which will be fraternal and 4 of which will be identical. Throughout history, it has been widely held that twins could only be born as either identical or fraternal; however, this notion changed in 2007 when the birth of two semi-identical twins were discovered in the U.S.
Related article: Conjoined Triplets (Is It Even Possible?)
Identical vs. Fraternal Twins
The way in which twins become identical or fraternal depends entirely upon what happens during fertilization of the mother’s egg. Identical, or monozygotic twins, arise from a single fertilized egg that has been split into two. Identical twins will therefore have identical genetic material, which accounts for why these twins are often indistinguishable from each other.