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Multiple births occur about once per 80 to 89 births. Twins occur once in 80 births, triplets once in 80 x 80 cases (6400), and quadruplets once in 80 x 80 x 80 cases (512,000). Heredity, the age of the mother, and racial factors appear to be of significance in that multiple births apparently occur more frequently in one family than in another, and to more women in their thirties than in their twenties.
Negroes have more twins than do whites, who in turn have more twins than do oriental women. Triplets are born to whites at a one to 10,200 ratio and to nonwhites at a 1 to 6200 ratio, this difference holding in both northern and southern states.
Identical twins develop from a single fertilized ovum that first divides and then separates. Each part then continues the process of cell division separately. Identical twins, consequently, have identical sets of chromosomes, are always of the same sex, and have a single placenta. If the cell mass does not make a complete separation in the identical-twinning process, the result is Siamese or joined twins. Fraternal twins develop from two separate ova, both of which are usually fertilized at relatively the same time. A single follicle may expel two or more mature ova, or ova in two or more follicles may develop to maturity simultaneously. Fraternal twins can be of the same or different sex, will have separate placentas, and will bear no more resemblance to one another than separately born siblings.
The birth of most twins follows the usual pattern of parturition, although there occasionally have occurred some interesting variations in the conception and birth process. For example, most twins arrive within a few minutes to an hour of each other; however, there are infrequent cases in which considerable time elapses between their births. In one instance, the babies arrived forty-eight days apart; in another, thirty, with one infant being born in December and the other in January. Mothers have given birth to twins of two different races, fathered, it follows, by two men, the conceptions having taken place within a short time of each other. At least one birth of twins has been recorded in which the babies, obviously fathered by different men, had different blood types.
Triplets may result from the fertilization of three different ova. More commonly, only two eggs are involved, one of which separates and then develops into identical twins. Quadruplets are for the most part the product of the fertilization of two ova, each of which separates and develops into a set of identical twins.
In multiple births, the percentage of males decreases with the number of children born. In one large sample study, among single births the percentage of males was 51.59; among twins, 50.85% were boys; among triplets, 49.54%; and among quadruplets, 46.48%. The explanation very likely is that from conception on, survival favors the female, and in multiple births the biological tendency observed in single births is presumably increased.1"7
Viable twin pregnancies will terminate about twenty-two days earlier than viable single pregnancies do, the average gestation period for twins being thirty-seven weeks.
Despite old wives' tales to the contrary, the future fertility of human twins, whether or not of the same sex, is no different from that of the singly born. Among lower animals, it might be noted, twins are in fact less fertile than animals born singly.
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