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.