Multiple birth

A multiple birth is the culmination of one multiple pregnancy, wherein the mother delivers two or more offspring. A term most applicable to placental species, multiple births occur in most kinds of mammals, with varying frequencies. Such births are often named 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 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.

Terms used for multiple births or the genetic relationships of their offspring:

Terms used for the number of offspring in a multiple birth:

In humans, the average length of pregnancy (two weeks fewer than gestation) is 38 weeks with a single fetus. This average decreases for each additional fetus: to thirty-six weeks for twin births, thirty-two weeks for triplets, and thirty weeks for quadruplets. With the decreasing gestation time, the risks from immaturity at birth and subsequent viability increase with the size of the sibling group. Only as of the twentieth century have more than four all survived infancy.

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