A multiple birth is when two or more babies are born from the same pregnancy. The most common types of multiple births are twins. There are two types of twins: identical and fraternal. Identical twins are caused by one egg being fertilized by one sperm. Sometimes after fertilization, the egg splits into two genetically identical halves. They have duplicate chromosomes, which leads to identical physical characteristics, such as hair color, eye color, and blood type. This type of twin accounts for about one in two hundred fifty births (Agnew, Klein & Robert, 17). Identical twins are always the same sex, although there has been about four very rare cases of opposite sex identical twins due to genetic abnormality (Agnew, Klein & Robert, 19). Identical twins share 100% of their genes, and are in reality, natural clones.
The second types of twins are fraternal twins. Fraternal twins are formed when two eggs released by the mother, and a different sperm fertilizes each egg (Twins). Fraternal twins are twice as common as identical twins. This type of twins can either be the same sex or opposite sex. They share up to 50% of their genes and are merely siblings born at the same time with no more genetically in common than siblings of different ages. It is also possible for fraternal twins to be conceived at different times. This happens when an egg is released even though the previous months egg was fertilized. If another man fertilizes the second egg, the fetuses are no more genetically alike than half0siblings (Twins.) Although there are only two types of twins, fraternal and identical, there are a variety of possible outcomes within these categories. Three distinct outcomes within the twin categories are mirror image twins, half-identical twins, and conjoined twins.
Some identical twins are mirror image twins. This is extremely rare and it occurs in about 25% of identical twins.