Multiple Birth
A multiple birth occurs where more than one fetus is carried to term in a single pregnancy. Different names for multiple birth are used, depending on the number of offspring. Common multiples are two and three, known as twins and triplets respectively. Twins, triplets and other multiple births occur to varying degrees in most animal species, although the term is most applicable to placental species.
There are two common types of multiple births, fraternal and identical. Identical siblings arise where one egg is fertilised and the resulting zygote splits into more than one embryo. Identical siblings therefore have the same genetic material. Fraternal siblings result from the fertilisation and implantation of more than one egg, so fraternal siblings are not genetically identical, and instead have the same genetic similarity any other full siblings do.
A very rare third type of twinning is believed to occur in which an unfertilized ovum splits into two identical cells prior to fertilization, creating a second complete ovum, called the "polar body". After splitting, both ova are then fertilized by different sperm. This results in twins who have received identical genes from their mother, but of the genetic material they received from their father, only about half of it is shared. Thus polar body twins have about 75% of their genes in common, less than the 100% shared by identical twins but more than the 50% shared by fraternal twins. They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins. However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins, so there has been no confirmation that such twins actually exist.
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