In earlier studies, scientists have attempted to identify genetic and environmental factors affecting the rate of multiple maternities among humans. We contribute to these studies by analysing the frequencies of multiple maternities in sibships containing triplets. Use of the Hellin transformation is included in evaluation of the triplet rate. Our results indicate greater frequencies of repeated multiple maternities in the sibships than expected, based on population frequencies. The excesses obtained are more marked in triplet maternities than in twin maternities. The transformed triplet rate shows results similar to the twinning rate. The findings also indicate that in families, the influence of maternal factors on the frequencies of multiple maternities is stronger than the influence of paternal factors.
