The frequency of living twins born in Southern Moravia in the period 1960-1970 was studied and found to be 0.67%. No differences in twinning rate due to the years, seasons, districts or towns with 10,000 and more inhabitants in comparison with other localities, were found. An interesting hitherto undescribed phenomenon of twin-birth clustering in space and time was observed.
Out of a total of 1025 twin births occurring in 11 South Moravian districts from 1972 to 1976, 638 occurred in the same time and space interval (P = 0.00037). The observed clustering suggests the existence of environmental factors influencing twinning rates.
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