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  • Frequency of Triplets and Triplet Zygosity Types among U.S. Births, 1964

  • Gordon Allen
  • Journal:
    Acta geneticae medicae et gemellologiae: twin research / Volume 37 / Issue 3-4 / October 1988
    Published online by Cambridge University Press:
    01 August 2014, pp. 299-306
    Print publication:
    October 1988
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  • The frequency of triplets in the U.S. white population may have reached an all-time low around 1964, at 78 sets per million deliveries. One-fourth of those were monozygotic as estimated by the difference method, or 18% by Bulmer's theoretical model. By 1983 the frequency of triplets had nearly doubled, the increase presumably occurring in dizygotic and trizygotic types. In Belgium most triplet pregnancies now result from artificial induction of ovulation, which is expected to occur mainly in older mothers. In the U.S., however, triplets have increased as much in young mothers as in older mothers, proportionally. This age distribution of the increase may be partly explained by a decrease in parity in older mothers since 1964.