Twins who share the same genetic makeup, also known as monozygotic twins. When a single fertilized egg divides in two separate embryos before the 8th day after conception, identical twins result. (If the division occurs after the 8th day, conjoined, or Siamese, twins result.) Identical twins share the same placenta, are always of the same sex, blood type and usually closely resemble one another. Twins have a higher risk of complications and premature birth. Identical twins happen in about 1 of every 250 births and make up 33% of all twin births. The incidence of identical twin pregnancies is very similar for all races and age groups.