Multiple birth

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A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple births are used, depending on the number of offspring. Common multiples are two and three, known as twins and triplets. These and other multiple births occur to varying degrees in most animal species, although the term is most applicable to placental species.

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.

Contents

Terminology

  • Monozygotic – multiple (typically two) fetuses produced by the splitting of a single zygote
  • Dizygotic – multiple (typically two) fetuses produced by two zygotes
  • Polyzygotic – multiple fetuses produced by two or more zygotes

Terms used for the order of multiple births in human beings are largely derived from the Latin names for numbers. Two offspring (twins) is the most common form, eight (octuplets) is the largest number ever successfully carried to full term with all children surviving.

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