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A multiple birth is the culmination of one multiple pregnancy, wherein the mother delivers two or more offspring, one for each fetus. A term most applicable to placental species, multiple births occur in most kinds of mammals, with varying frequencies. Such births are often named also according to the number of offspring, as in ''twins'' and ''triplets''. In non-humans, the whole group may also be referred to as a ''litter'', and multiple births may be more common than single births. Multiple births in humans are the exception, and can be exceptionally rare in the largest mammals. Each single fertilized egg (zygote) may produce a single embryo, or it may split into two or more embryos, each carrying the same genetic material. Fetuses resulting from different zygotes are called ''fraternal'' and share only 50% of their genetic material, as ordinary full siblings from separate births do. Fetuses resulting from the same zygote share 100% of their genetic material, and are hence called ''identical'' and are always of the same sex (except in cases of sex reassignment or, much more rarely, Turner syndrome). A multiple pregnancy may be the result of the fertilization of a single egg that then splits to create identical fetuses, or it may be the result of the fertilization of multiple eggs that create fraternal fetuses, or it may be a combination of these two. A multiple pregnancy from a single zygote is called ''monozygotic'', from two zygotes is called ''dizygotic'', or from two or more zygotes is called ''polyzygotic''. Similarly, the siblings themselves from a multiple birth may be referred to as monozygotic if they are identical, or as polyzygotic if they are fraternal. ==Terminology== Terms used for multiple births or the genetic relationships of their offspring: *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 number of offspring in a multiple birth: *Two offspring - twins *Three offspring - triplets *Four offspring - quadruplets *Five offspring - quintuplets *Six offspring - sextuplets *Seven offspring - septuplets *Eight offspring - octuplets *Nine offspring - nonuplets 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Multiple birth」の詳細全文を読む スポンサード リンク
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