the delivery of more than one offspring in a single birth event. In most mammals the litter size is fairly constant and is roughly correlated with, among other features, body size, gestation period, life span, type of uterus, and number of teats. For example, a large mammal with a normal pregnancy of more than 150 days, a life span of more than 20 years, a simple (or unicornuate) uterus, and two breasts rarely bears more than one young (a singleton) at a birth. The great majority of primates, humans included, fit into this category; multiple births among them are exceptional and occur with decreasing frequency from twins on.

Twinning refers to the process that leads to the production of more than one offspring at one birth. In humans the most frequent type of twinning results in the birth of two babies (twins (twin)) at once, although repetition of twinning may lead to triplets, quadruplets, or other multiples of one.

The two major types of twins are identical twins and fraternal twins. Identical twins are two individuals that have developed from a single egg fertilized by a single sperm. This fertilized egg is called a zygote. At a relatively early stage in its growth, the zygote splits into two separate cell masses which go on to become embryos (embryo); these embryos are genetically identical to each other and are always of the same sex. Three-fourths of such embryo pairs share a common placenta. Since they both developed from a single zygote, such twins are called monozygotic (MZ) twins. A zygote's atypical separation into two independent embryonic structures can occur at any of several growth stages. Its incomplete or late division into two cell masses results in conjoined twins (conjoined twin), formerly known as Siamese twins. MZ twins usually show a striking physical resemblance to one another. It should be noted that even though hereditary characteristics such as eye colour and hair colour and texture are the same in MZ twins, these traits as well as the majority of physical characteristics may be modified during embryonic development. Identical twins may therefore not truly be “identical”; the correspondence between such twins is closer to what would be expected between the right and left sides of a single individual, which vary slightly from one another.

Fraternal twins develop from two separate eggs that have been fertilized by two separate sperm. Such egg pairs stem from multiple ovulations (ovulation) in the same cycle. Each egg develops singly, is fertilized singly, and forms its own individual placenta. Since fraternal twins develop from different zygotes, they are called dizygotic (DZ) twins. DZ twins may be of the same sex or of different sexes, and they are not genetically identical to one another. Fraternal twins bear the resemblance of ordinary siblings.

The occurrence of DZ twinning varies among racial groups: multiple births are most common among people of African descent, less common among those of European descent, and least common among people of Asian descent. DZ twinning is most common in older mothers (up to about 40 years of age) and tends to recur in families having a history of multiple births, indicating that a hereditary factor may be involved. MZ twinning, on the other hand, occurs randomly in all races and follows no discernible genetic pattern; it tends to occur more frequently in older mothers, however.

In the United States, Canada, and Great Britain about 1 out of every 83.4 births is a twin birth. Reported frequencies throughout the world vary from about 1 in 70 to 1 in 145 births; however, source records from many areas are unreliable. In North America DZ twins are more frequent than MZ twins, the relative proportions being 71.8 to 28.2; similar proportions hold among other populations of European origin. Excluding the use of fertility drugs, approximate figures for the frequency of multiple births are 1 in 80 births for twins, 1 in 6,400 for triplets, 1 in 512,000 for quadruplets, 1 in 40,960,000 for quintuplets, and so on.

As with twins, the other types of multiple births may or may not be monozygotic. Any combination may occur; e.g., triplets may be derived from a single zygote (MZ triplets); from two zygotes, one of which divided to produce MZ twins while the other developed as a singleton (the three constituting DZ triplets); or from three separate zygotes (trizygotic, or TZ, triplets). Similarly, quadruplets may originate from one up to four zygotes, and quintuplets may derive from one to five zygotes. After being carefully studied, the Canadian Dionne quintuplets (born in 1934) were shown to be a one-zygote set.

Since the 1960s a substantial increase in the number of births of quintuplets, sextuplets, and even septuplets has occurred; this unprecedented phenomenon stems from use by the female of fertility (infertility) drugs, which cause the simultaneous ovulation of many eggs, and in vitro fertilization, a procedure in which several fertilized eggs are implanted in a woman at one time. Because they are often born premature (premature birth and postmature birth), multiple order babies are at greater risk of infant death and chronic health problems than those who are singletons. The first confirmed birth of octuplets was reported from Mexico City in March 1967; none of the four boys and four girls born prematurely survived more than 14 hours. The first recorded set of nonuplets was born June 13, 1971, when an Australian woman gave birth to five boys and four girls (two boys were stillborn). The remaining seven infants died within a week.

Because MZ twins carry identical genes, they have been used extensively in medical and psychological research. By comparing sets of MZ twins to carefully matched control sets of DZ twins (the “twin method”), researchers have attempted to elucidate the relative importance of heredity versus environment in the development of certain diseases, in the formation of the personality, and in intelligence. Such studies have, for example, demonstrated a strong genetic component in the development of schizophrenia, mental depression, and obesity, as well as in the susceptibility to such infectious diseases as tuberculosis.

Twin studies, especially those that have involved identical twins who have been reared apart, have provided intriguing (but not conclusive) evidence on the importance of heredity in establishing the personality and in determining intelligence. Because of the unique value of twin studies, many nations keep registries of twins.