A multiple birth is the culmination of one multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate 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.
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 ("non-identical") fetuses, or it may be a combination of these factors. A multiple pregnancy from a single zygote is called monozygotic, from two zygotes is called dizygotic, or from three 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 dizygotic (in cases of twins) or polyzygotic (for three or more siblings) if they are fraternal, i.e., non-identical.
Each fertilized ovum (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 hence are called identical. Identical twins are always the same sex.
Terminology
Terms used for the number of offspring in a multiple birth, where a number higher than three ends with the suffix -uplet: two offspring – twins three offspring – triplets four offspring – quadruplets five offspring – quintuplets six offspring – sextuplets seven offspring – septuplets eight offspring – octuplets nine offspring – nonuplets ten offspring – decuplets
Terms used for multiple births or the genetic relationships of their offspring are based on the zygosity of the pregnancy: Monozygotic – multiple (typically two) fetuses produced by the splitting of a single zygote Polyzygotic – multiple fetuses produced by two or more zygotes: Dizygotic – multiple (typically two) fetuses produced by two zygotes Trizygotic – three or more fetuses produced by three zygotes Sesquizygotic – an egg which is fertilized by 2 sperms, which produce 2 fetuses
Multiple pregnancies are also classified by how the fetuses are surrounded by one or more placentas (chorionicity) and amniotic sacs (amnionicity).
Human multiple births
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.
Recent history has also seen increasing numbers of multiple births. In the United States, it has been estimated that by 2011, 36% of twin births and 78% of triplet and higher-order births resulted from conception by assisted reproductive technology.
Twins
Twins are by far the most common form of multiple births in humans. The U.S. Centers for Disease Control and Prevention report more than 132,000 sets of twins out of 3.9 million births of all kinds each year, about 3.4%, or 1 in 30. Without fertility treatments, the probability is about 1 in 60; with fertility treatments, it can be as high as 20-25%.
Dizygotic (fraternal) twins run in families; however, the man does not influence the chances of the woman having twins, even if there are twins in his family. The hyperovulation gene is the cause for having twins and is only a factor for the mother. Although the man does not influence the woman's chances of having twins, he could influence his children's chances for having twins; if he carries the gene for hyperovulation and has a daughter, she may also have the gene for hyperovulation and then have twins herself.
Monozygotic (identical) twins do not run in families. The twinning is random, due to the egg splitting, so all parents have an equal chance of conceiving identical twins.
