7 Types of Twins – and What Makes Them All Different

While they usually look almost exactly alike, this isn’t always the case. Regardless of physical appearance, however, they still share all the same genetic material and are genetically identical, explains Jamie Stanhiser, MD, MSCR, a double board-certified physician who specializes in reproductive endocrinology and infertility. According to Stanhiser, while twins occur in about 3 percent of pregnancies, identical twins are a smaller subset, happening in approximately one third of twin pregnancies.