There IS LITTLE agreement as to the incidence of multiple births among the many studies of their occurrence. This is not unexpected, however, since it has been postulated that the frequency of multiple births is influenced by a number of factors, among them the fertility of the people, which varies in different periods, multiparity, frequency of pregnancy, age of the mother, climate, race, and hereditary factors. Each study, then, can have significance only in terms of the specific population group examined; the results cannot be extended to a larger group and retain their validity. Table 1 provides an example of the wide variation in results of these studies.
For practical purposes or, more precisely, for the purpose of the practicing physician, the formula devised by Hellin2 is adequate, even though erroneous. He declared that the ratio of single births to twin births was about the same as that of
Berbos JN, King BF, Janusz A. Quintuple PregnancyReport of a Case. JAMA. 1964;188(9):813–816. doi:10.1001/jama.1964.03060350039010
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