Identical twins are formed when one egg is fertilized by a single sperm cell and then divides within the first few days of pregnancy. If division of the egg occurs after 8 days, this can result in conjoined twins, also known as Siamese twins. Twins produced by this process are referred to as identical since their DNA sequence is identical. While the twins are identical genetically, this often does not apply to the actual physical or behavioral traits. Generally their traits are similar, however they can differ. Fraternal twins are formed when two eggs are independently fertilized by two sperm cells. Fraternal twins do not have identical DNA but will have half of their DNA in common, just like siblings of the same parents that are born at different times. Fraternal twins can either be of the same sex or different sex. The occurrence of fraternal twins varies substantially in frequency over the world. They are common in the pregnancies of older women, especially common in Africans, and relatively rare among those of Asian heritage.