Nearly one in every hundred deliveries is a twin birth. Triplet, quadruplet, and higher order deliveries occur far less frequently–only 1 in 10,000 deliveries. This article concerns only twins.
Twins are of two kinds: identical and fraternal. Biologists call the former monozygotic twins and the latter dizygotic twins, in reference to their different origins.
Identical (monozygotic) twins derive from a single fertilized egg, or zygote, that has divided in two in the course of its development. The two resulting embryos are genetically identical, which explains the close resemblance of monozygotic twins. They are always of the same sex.
Fraternal (dizygotic) twins derive from the ovulation and fertilization of two different ova during the same menstrual cycle. Each of these ova is fertilized by a spermatozoon and the twins resulting from these two eggs or zygotes are no more similar, from a genetic point of view, than ordinary brothers and sisters. Fraternal twins can be of the same sex or male and female, both variants occurring with equal frequency.
Fraternal and identical twins thus correspond to two distinct biological processes, and their incidence depends on different factors.
The Twinning Rate
The twinning rate is the proportion of twin deliveries in the total number of deliveries.
Identical twin deliveries occur at the rate of 3.5 to 4 per 1,000, regardless of the mother's age, birth order, or ethnic or geographic origin. The same proportion has been observed among all mammals, except for some armadillos that systematically give birth to monozygotic quadruplets or octuplets. All women seem to run the same risk of having identical twins, whether or not they have previously given birth to twins.
In contrast to identical twin births, the proportion of fraternal twin births is extremely variable. The main factors influencing these variations are as follows.
Age of the mother. Beginning with a near zero level at puberty, the proportion steadily increases up to age 37, where it reaches its maximum level, then rapidly decreases back to zero level by the time of menopause. This variation corresponds to that of the Follicle Stimulating Hormone (FSH), which ensures the development of the ova. (The drop in the fraternal twin rate after the age of 37 could be due to weaker ovarian function and to the higher mortality of fertilized eggs as menopause draws nearer.)
Order of birth. Controlling for age of mother, the fraternal twinning rate increases with every childbirth. Birth order is nevertheless less influential than age.
Geographic or ethnic origin. The same variations by mother's age and order of birth are observed everywhere, but the frequency of twinning differs by region. Controlling for age and birth order, the fraternal twinning rate in sub-Saharan Africa is two times higher than in Europe, and four to five times higher than in China or Japan. These differences are partly linked to hormonal differences of genetic origin. Hence, for example, the twinning rate of African-Americans in the United States lies between the European and the African rates.
Individual and family characteristics. Some women may have several sets of fraternal twins; this predisposition to twin pregnancies is partly genetic and can be observed among the sisters and daughters of women who have had twins.
The Influence of Sterility Treatments
In France, in the first half of the twentieth century, the incidence of twin deliveries was about 11 per 1,000, a proportion which did not significantly vary, except during World War I, when the twinning rate temporarily rose. (See Figure 1.) In the 1960s, the proportion of twin deliveries declined, reaching a low 8.9 per 1,000 in 1972. The rate then began to climb again and by 1987, it had risen back to the level of the first half of the century. However, the upward trend did not stop there, and even gained momentum: By 1998, the twinning rate had reached 14.7 per 1,000, a 65 percent increase from 1972. The same downward and upward trends were observed in most developed countries.
A partial explanation for these trends is variation in the mean age of mothers. In France in the 1950s, for example, the mean age was close to 28; it fell to 26.5 in 1977. A rapid increase followed and, by the end of the 1990s, it exceeded 29. However, the most important factor in the steep rise in twinning rates since the 1970s has been the expanded use of sterility treatments. Twinning rates rose especially in developed countries, where such treatments are most available, and particularly among older women, who are more likely to utilize them.
French physicians began to prescribe hormones to stimulate ovulation in 1967. The treatments became so popular that by 2000 some 400,000 menstrual cycles were being stimulated each year. By comparison, the total number of births in France in 2000 was 780,000. In addition, at the beginning of the twenty-first century some 40,000 in vitro fertilization (IVF) procedures are performed per year. In order to improve the likelihood of IVF success, physicians often implant several ova or several embryos at once–2.5 on average in 1997–resulting in a high probability of multiple births. Almost one out of four IVF pregnancies leads to the birth of twins, as opposed to one in 100 for natural pregnancies.
Mortality of Twins
In all parts of the world, the mortality rate of twin babies is much higher than that of singletons, due to their often low birth weight, their tendency to be premature, and more frequent complications at birth. The risk of giving birth to a stillborn twin is three to four times as high as that for a singleton. The mortality rate of twins born alive is also higher than that of singletons. In the first month following birth, the mortality rate for a twin is five to seven times higher than for a singleton, both in countries where infant mortality rates are high and in countries where the rate is low. After the first month, the gap decreases, but, regardless of the overall level of mortality, the mortality rate of twins remains two to three times that of singletons through the first year of life and continues to exceed that of singletons throughout childhood.
See also: Reproductive Technologies: Modern Methods.
Bulmer, M. G. 1970. The Biology of Twinning in Man. Oxford: Clarendon Press.
FIVNAT. 1995. "Grossesses Multiples." Contraception Fertilité Sexualité 23(7–8): 494–497.
FIVNAT. 1997. "Bilan Général Fivnat, 1997." Contraception Fertilité Sexualité 26(7–8): 463–465.
Institut National de la Statistique et des Études Économiques (INSEE). Various years. La Situation Démographique. Paris: INSEE.
Office for National Statistics, Great Britain. 1995. Mortality Statistics in England and Wales. Childhood, Infant and Perinatal. London: The Stationery Office.
Pison, Gilles. 1992. "Twins in Sub-Saharan Africa: Frequency, Social Status and Mortality." In Mortality and Society in Sub-Saharan Africa, ed. Etienne van de Walle, Gilles Pison, and Mpembele Sala-Diakanda. Oxford: Clarendon Press.
Pison, Gilles. 2000. "Nearly Half of the World's Twins Are Born in Africa." Population et Sociétés 360: 1–4. <http://www.ined.fr/englishversion/publications/pop_et_soc/index.html>.
Aristotle long ago aptly noted that multiple births are “praeter naturam,” that is, “outside nature’s normal course.” Being outside the normal, their reception in society is also different than that afforded to the single child. In many parts of the world, they are seen as good luck and have become absorbed into the local mythology. This was the case in ancient Rome and Greece, for example, and also in Mesoamerica. In other parts of the world, as was often the case in some but not all parts of Africa, they were not welcomed, being seen as evidence of maternal marital infidelity.
The fact that Aristotle’s dictum remains correct centuries later is based on two inescapable facts: First, all multiple births are high-risk pregnancies in contrast to singletons (single births), and second, as the human female is genetically programmed to have a single child, both mother and child suffer these risks. This latter statement relates to the facts that under normal circumstances 98 percent of human pregnancies deliver one child only and that the distensibility of the human uterus is clearly limited. This reality is probably the greatest reason that higher-order multiples deliver preterm.
Twins are the most common form of multiple pregnancies, followed by triplets and then the rare occurrences of quadruplets and quintuplets. Prior to modern technologies for assisted reproduction, the incidence of twins was 1 in 89 deliveries, of triplets 1 in 892, and of quadruplets 1 in 893. All that changed after 1975 when clomiphene citrate became available; even more drastic changes followed post-1985 with the introduction of in vitro fertilization and embryo transfer. The rate of twin births has more than doubled, while that of triplets has tripled. U.S. trends were duplicated in many other localities that have access to modern treatments for infertility.
Twins are either identical or fraternal, though neither term is correct in that identical twins, derived from a single fertilized ovum, are never truly identical at the molecular genetic level, and fraternal twins, derived from two fertilized ova, may be female as well as male. More precise terms are monozygotic (MZ), meaning “one-egg,” and dizygotic (DZ), meaning “two-egg,” which both refer to embryological origins. Natural twins, the most frequent form of multiples, occur most commonly in Africans and least commonly in Asians, with the Euro-American Caucasian population in-between. The exact reasons for this have never been clarified, but it is known that the majority of twins in African populations are dizygotic, whereas the majority in Asian populations are monozygotic. In Caucasians, approximately two-thirds of naturally occurring twins are DZ and one third are MZ. In Africans, DZs far outnumber the MZs, presumably because of frequent double ovulation. In Asians, MZs predominate due to the relative scarcity of double ovulation. These racial variations have always been regarded as fact within the literature on multiples and have never been interpreted as signs of racial superiority/inferiority.
A third type of twins is the so-called Siamese or conjoined variety, named after the twins Eng and Chang, from Siam (now Thailand). These occur in somewhere between 1 in 50,000 and 1 in 100,000 births. Their cause is due to partial division of one zygote after thirteen days post fertilization. Female conjoined twins are more common than males in a ratio of 1.6:1, for reasons that are not entirely clear. It has been suggested that when conjoined twinning occurs at the embryonic plate, it is more lethal in males, though this has not been proven.
The prime cause of morbidity and mortality among all multiples is preterm delivery and low birth weight, which go hand in hand. Whereas the normal gestational length for singletons is forty weeks, the median gestational length is thirty-seven weeks for twins, thirty-three weeks for triplets, thirty-one for quadruplets, and twenty-nine for quintuplets. The increased numbers of preterm multiples raises U.S. rates of cerebral palsy by at least 7 percent. Mothers of multiples face five risks: anemia, postpartum bleeding, PIH (high blood pressure), polyhydraminos (excess water in one of the amniotic sacs), and preterm labor.
SEE ALSO Twin Studies
Blickstein, Isaac, and Louis G. Keith, eds. 2001. Iatrogenic Multiple Pregnancy. London: Parthenon.
Blickstein, Isaac, and Louis G. Keith, eds. 2005. Multiple Pregnancy: Epidemiology, Gestation, and Perinatal Outcome. 2nd ed. London: Parthenon.
Keith, Louis G., and Isaac Blickstein, eds. 2002. Triplet Pregnancies and Their Consequences. London: Parthenon.
Throughout history there have been strongly held cultural and religious beliefs about twins. Attitudes to them varied widely from fear, or hostility, to worship and belief in their supernatural powers (e.g., to induce fertility). Misconceptions about the biology of twins have also been rife. In mid-seventeenth-century Europe it was still thought that boy and girl twins could not coexist in the womb because of the horrorincestus, that two children meant two fathers, and that infertility was inevitable in the female of a boy-girl pair.
Sir Francis Galton (1876) first recognized the potential of twins in research on the effects of heredity and environment on human development. Essential to this research was the new understanding of the two distinct types of twins—monozygotic (MZ) who, arising from the splitting of one fertilized egg, have the same genetic makeup and dizygotic (DZ) twins who share only half of their genes, arising from two separately fertilized ova. (Zygosity was initially, and unreliably, determined by comparing the physical features of twins but DNA analysis later became the preferred method.)
The DZ, but not the MZ, twinning rate varies in different ethnic groups. Since the 1950s all developed countries have seen the same trends. Following a decline between 1950 and the late 1970s, there was a steady increase in DZ twinning after 1980, due largely to the increasing use of infertility treatments such as ovulation stimulating drugs or multiple embryo transfers following in vitro fertilization (IVF). Clomiphene was introduced in the 1960s and injectable gonadotrophins a decade later. The first IVF baby was delivered in 1978 in the United Kingdom, in 1980 in Australia, and in 1981 in the United States. The first IVF twins were born in 1981. There has been a much more rapid rise in the number of higher multiple births with triplet rates increasing three to six fold since the mid 1980s. Triplet rates in some IVF units rose to over 6 percent of pregnancies. At the beginning of the twenty-first century triplet rates started to decline in a few countries as infertility clinics become more aware of the potential hazards of multiple births. Nevertheless, the twinning rate continued to increase, except in the few countries that adopted a policy of single embryo transfer following IVF. Ovulation induction continued to result in large higher multiple pregnancies with the largest birth by 2003 being of nonoplets (9) and the largest set of surviving children, septuplets (7).
MZ twinning rates are quite different: they were constant worldwide at 3.5 per 1000 births until the 1990s when an unexplained, slight rise was detected. The causes of MZ twinning are unknown but six to twelve times the expected number are found amongst twins and triplets resulting from ovulation inducing forms of treatment for infertility, whether or not these are accompanied by IVF.
By the 2000s, a set of six multiple birth children had reached adulthood in good health and with normal development. However, in general, the degree of prematurity and low birthweight, with all the associated neonatal complications, mortality and long term morbidity, increases with the number of fetuses. Many previously infertile couples face the painful choice of a multifetal pregnancy reduction to twins or continuing a pregnancy which carries a high risk of death or disability for some or all of their children.
See also: Conception and Birth; Fertility Drugs.
Blickstein, Isaac, and Louis Keith. 2001. Iatrogenic Multiple Pregnancy. Carnforth, UK: Parthenon Publishing Group.
Bryan, Elizabeth. 2002. "Loss in Higher Multiple Pregnancy and Multifetal Pregnancy Reduction." Twin Research 5: 169–174.
Corney, Gerald. 1975. "Mythology and Customs Associated with Twins." In Human Multiple Reproduction, ed. Ian MacGillivray, Percy P. S. Nylander, and Gerald Corney. London: WB Saunders.
Galton, Francis. 1876. "The History of Twins as a Criterion of the Relative Powers of Nature and Nurture." Journal of the Anthropological Institute 5: 391–406.
Gedda, Luigi. 1961. Twins in History and Science. Springfield, IL: Charles Thomas.