multiple birth, bringing forth of more than one offspring at birth. Although many smaller mammals bear several young at a time, multiple births are relatively uncommon in humans and other primates. Twinning, the process that leads to the production of more than one offspring, results in twins, and with decreasing frequency, triplets, quadruplets, quintuplets, sextuplets, septuplets, and octuplets.

In the one-egg, or identical, type of twinning, a single fertilized ovum divides to form two complete organisms. Such twins are always of the same sex, are usually extraordinarily similar in physical appearance, and have identical blood-group types. Twinning to form one-egg identical twins usually takes place early in pregnancy. If considerable development has taken place before the twinning occurs, there may be an incomplete separation of the two embryos resulting in conjoined offspring called Siamese twins.

Fraternal twins, which are more common than identical twins, are those that develop from two separate ova, each fertilized by a sperm. Fraternal twins may or may not be of the same sex and need not resemble each other more than do any other two offspring of the same parents. In the United States twins occur once in approximately 40 births. In rare cases, non-identical embryos can fuse in the womb to produce a condition called chimerism, in which some of the individual's cells come from one of the embryos and the rest of the cells come from the other, genetically distinct embryo.

The incidence of multiple-egg births is in part genetically determined, varying according to race and family tendencies; and it is also influenced by external factors, i.e., the incidence increases with increasing age of the mother and the number of children she has already borne. One-egg, or identical, twinning occurs with the same frequency in all women, regardless of race, age, or other factors. There is evidence from comparative biology that deleterious factors in the environment of the newly fertilized ovum, such as a reduction in oxygen, increase the likelihood of one-egg twinning. Fertility drugs such as clomiphene, which are used when the cause of infertility is lack of released ova, sometimes cause several ova to be released and fertilized simultaneously. The use of these drugs has led to a rise in the incidence of multiple births, including sextuplets, septuplets, and octuplets.

Birth of more than one child from one pregnancy. Twins are most common, born in 1 of about every 80 pregnancies. Identical twins develop from a single fertilized egg, which splits into two genetically identical embryos (though physical traits may be modified during their development); they occur randomly but are more likely in older mothers. Incomplete or late division results in conjoined twins. Fraternal twins develop from two eggs fertilized by two sperm and are no more genetically alike than are other siblings. Most common among persons of African ancestry and least common among those of Asian ancestry, fraternal twins seem to run in families. Repeated twinning produces triplets, quadruplets, and so on; these multiples may be identical, fraternal, or a combination. The use of fertility drugs has increased the number of high-order multiple births. Medical and psychological “twin studies” compare fraternal and identical twins to learn about genetic influences on various characteristics and diseases.

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