Aspects of the increasing triplet rate in Australia

Abstract

Summary: The sex composition of 862 sets of Australian triplets was independent of decade and State of birth during the period 1920–69. The triplet rate increased by 40% during the 50 years in contrast with declining rates reported elsewhere. The increase was due to dizygotic and trizygotic types, together but not individually, and occurred despite an 8·6% decrease in the average national maternal age over the same period. Adjustment for age led to standardized individual zygosity rates, estimates of which were compared with reported values obtained elsewhere. The dependence of zygosity rates on maternal age was shown to rise progressively as the number of ova released in triplet formation increases.