A multiple birth is the culmination of a 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 almost always the same sex; however, in rare circumstances, chromosomal mutations can lead to different sexes being expressed.
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 (2 weeks fewer than gestation) is 38 weeks with a single fetus. This average decreases for each additional fetus: to 36 weeks for twin births, 32 weeks for triplets, and 30 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 in the twentieth century have more than four children 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. Compared to other multiple births, twin births account for 97% of them in the US. Without fertility treatments, the probability is about 1 in 60; with fertility treatments, it can be as high as 20-25%.
A hyperovulation gene in the mother can cause dizygotic (fraternal) twins. Although the father's genes do not influence the woman's chances of having twins, he could influence his children's chances of having twins by passing on a copy of the hyperovulation gene to them.
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.
Triplets
Triplets can be either fraternal, identical, or a combination of both. The most common are strictly fraternal triplets, which come from a polyzygotic pregnancy of three eggs. Less common are triplets from a dizygotic pregnancy, where one zygote divides into two identical fetuses, and the other does not. The least common are identical triplets, three fetuses from one egg. In this case, sometimes the original zygote divides into two, and then one of those two zygotes divides again, but the other does not, or the original zygote divides into three.
Triplets are far less common than twins, according to the U.S. Centers for Disease Control and Prevention, accounting for only about 4,300 sets in 3.9 million births, just a little more than .1%, or 1 in 1,000. According to the American Society of Reproductive Medicine, only about 10% of these are identical triplets: about 1 in ten thousand. Nevertheless, only 4 sets of identical triplets were reported in the U.S. during 2015, about one in a million. According to Victor Khouzami, Chairman of Obstetrics at Greater Baltimore Medical Center, "No one really knows the incidence".
Identical triplets or quadruplets are very rare and result when the original fertilized egg splits and then one of the resultant cells splits again (for triplets) or, even more rarely, a further split occurs (for quadruplets). The odds of having identical triplets is unclear. News articles and other non-scientific organizations give odds from one in 60,000 to one in 200 million pregnancies.
Quadruplets Quadruplets are much rarer than twins or triplets. One famous set of identical quadruplets was the Genain quadruplets, all of whom developed schizophrenia. Quadruplets are sometimes referred to as "quads".
In July 2026, a woman gave birth to identical quadruplet daughters in Brisbane, Australia. In what was described as an extremely rare occurrence, the babies were conceived naturally, split from a singular fertilised egg, and all shared a single placenta. They were delivered by caesarean section at 28 weeks and four days gestation at the Royal Brisbane and Women's Hospital.
Quintuplets
Quintuplets occur naturally in 1 in 55,000,000 births. The first quintuplets known to survive infancy were the identical female Canadian Dionne quintuplets, born in 1934. Quintuplets are sometimes referred to as "quins" in the UK and "quints" in North America. A famous set of all-girl quintuplets are the Busby quints from the TV series OutDaughtered.
Sextuplets
Born in Liverpool, England, on November 18, 1983, the Walton sextuplets were the world's first all-female surviving sextuplets, and the world's fourth known set of surviving sextuplets. Another well-known set of sextuplets is the Gosselin sextuplets, born on 10 May 2004, in Hershey, Pennsylvania. Reality television shows Jon & Kate Plus 8 and later Kate Plus 8 have chronicled the lives of these sextuplets. Other shows of this nature include Table for 12 and Sweet Home Sextuplets.
Very high-order multiple births In 1997, the McCaughey septuplets, born in Des Moines, Iowa, became the first septuplets known to survive infancy. The first surviving set of octuplets on record are the Suleman octuplets, born in 2009 in Bellflower, California. In 2019, all 8 children celebrated their 10th birthday. Multiple births of as many as 9 babies have been born alive; In May 2021, the Cissé nonuplets were born in Morocco to Halima Cissé, a 25-year-old woman from Mali. , two years since their births, all 9 are still living and reportedly in good health.
The list of multiple births covers notable examples.
Causes and frequency
The frequency of N multiple births from natural pregnancies has been given as approximately 1:89N−1 (Hellin's law) and as about 1:80N−1. This gives: 1:89 (= 1.1%) or 1:80 (= 1.25%) for twins 1:892 (= 1:7921, about 0.013%) or 1:802 (= 1:6400) for triplets 1:893 (= approx. 0.000142%, less than 1:700,000) or 1:803 for quadruplets
North American dizygotic twinning occurs about once in 83 conceptions, and triplets about once in 8000 conceptions. US figures for 2010 were: Twins – 132,562 (3.31%) Triplets – 5,503 (0.14%) Quadruplets – 313 (0.0078%) Quintuplets and more – 37 (0.00092%)
Human multiple births can occur either naturally (the woman ovulates multiple eggs or the fertilized egg splits into two) or as the result of infertility treatments such as in vitro fertilization (several embryos are often transferred to compensate for lower quality) or fertility drugs (which can cause multiple eggs to mature in one ovulatory cycle).
For reasons that are not yet known, the older a woman is, the more likely she is to have a multiple birth naturally. It is theorized that this is due to the higher level of follicle-stimulating hormone that older women sometimes have as their ovaries respond more slowly to FSH stimulation.
The number of multiple births has increased since the late 1970s. For example, in Canada between 1979 and 1999, the number of multiple-birth babies increased 35%. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher-order births much rarer still. Much of the increase can probably be attributed to the impact of fertility treatments, such as in-vitro fertilization. Younger patients who undergo treatment with fertility medication containing artificial FSH, followed by intrauterine insemination, are particularly at risk for multiple births of higher order.
Certain factors appear to increase the likelihood that a woman will naturally conceive multiples. These include: mother's age: women over 35 are more likely to have multiples than younger women mothers' use of fertility drugs: approximately 35% of pregnancies arising through the use of fertility treatments such as IVF involve more than one child
Women conceiving multiples over the age of 35 increase the risk of having fetuses with certain conditions and complications that are not as common in women who are pregnant.
The increasing use of fertility drugs and consequent increased rate of multiple births has made the phenomenon of multiples more frequent and hence more visible. In 2004 the birth of sextuplets, six children, to Pennsylvania couple Kate and Jon Gosselin helped them to launch their television series, originally Jon & Kate Plus 8 and (following their divorce) Kate Plus 8, which became the highest-rated show on the TLC network.
Risks Premature birth and low birth weight Babies born from multiple-birth pregnancies are much more likely to result in premature birth than those from single pregnancies. 51% of twins and 91% of triplets are born pretermQuadruplets and over are pretty much impossible to have a full-term pregnancy., compared to 9.4% in singletons. Note: This study was done by looking at the 1991–1995 U.S. Natality Files, which were received from the National Center for Health Statistics 14% of twins and 41% of triplets are even born very preterm, compared to 1.7% in singletons.
Drugs known as beta-mimetics can be used to relax the muscles of the uterus and delay birth in singleton pregnancies. There is some evidence that these drugs can also reduce the risk of preterm birth for twin pregnancies, but existing studies are small. More data is required before solid conclusions can be drawn. Likewise, existing studies are too small to determine if a cervical suture is effective for reducing prematurity in cases of multiple birth.