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MULTIPLE BIRTHS

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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.

FIGURE 1

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.

BIBLIOGRAPHY

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.

INTERNET RESOURCE.

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>.

GILLES PISON