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Multiple Birth, birth of more than one offspring at a time, occurring regularly in most mammals, and uncommonly in some of the larger ones, such as cattle, horses, and humans. In humans, the tendency to bear more than one offspring is hereditary, and a mother who gives birth to twins is likely to have additional twins. Similarly, a twin-bearing mother stands a greater chance of producing triplets. The recent use of hormones and drugs to treat female infertility has increased the incidence of multiple birth, sometimes resulting in premature delivery of five or six offspring.

Multiple births in human beings arise either from the simultaneous impregnation of more than one ovum (egg) or from the impregnation of a single ovum that divides into two or more parts, each of which develops into a distinct embryo. Plural offspring developing from a single egg are known as identical; they are always of the same sex, resemble one another very closely, and have similar fingerprints and blood types. Offspring produced from separate ova are fraternal; not necessarily of the same sex, they have the usual family resemblance of brothers and sisters.

Twins are the most frequent form of multiple birth in humans, and identical twins occur only one-fourth as frequently as fraternal twins. Triplets are about 87 times rarer than twins, and quadruplets about 87 times rarer than triplets. Quintuplets occur in about 1 out of 8 million pregnancies. These figures are for spontaneously occurring pregnancies. When ovulation has been induced by drugs, the incidence rises. Multiple births may be achieved artificially by implanting in the uterus several ova fertilized in vitro.

The prenatal and infant mortality rate in multiple pregnancies is higher than that in single gestations. The danger of premature birth increases progressively with the number of offspring involved. The first quintuplets in medical history to survive were the Dionne quintuplets.