Multiple Birth

Multiple birth siblings are either monozygotic or dizygotic. The former result from a single fertilized egg or zygote splitting into two or more embryos, each carrying the same genetic material. Siblings created from one egg are commonly called identical. Since identical multiples share the same genetic material, they are almost always the same sex. Dizygotic or fraternal multiples instead result from multiple ova being ripened and released in the same menstrual cycle by a woman's ovaries, which are then fertilized to grow into multiples no more genetically alike than ordinary full siblings. Multiples called polyzygotic represent some combination of fraternal and identical siblings. For example, a set of triplets may be composed of identical twins from one egg and a third sibling from a second egg.
The most common form of multiple births for humans is twins. Many placental species give birth to multiples as a matter of course, with the resulting group called a litter.