Basic Facts: Science of Multiple Births
A multiple birth occurs where more than one fetus exits the womb in a single pregnancy. Different names for multiple birth are used, depending on the actual multiple. 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 (dizygotic) and identical (monozygotic). 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 an analogous genetic similarity of any brothers and sisters.
A very rare third type of twinning is now believed to occur when an unfertilized ovum splits into two equal parts prior to fertilization, creating a second complete ovum, called the "polar body". After splitting, both ovum are each then fertilized by different sperm. This results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm).
Polar body twins share about 75% of their genetic markers, less than identical twins but more than 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.
The most common form of human multiple birth is twins (two babies), but the typical order of multiple births in other placental species differs dramatically. Some species give birth to multiple offspring as a matter of course and the resulting group of offspring is called a litter.
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