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SummaryBirth records of Hutterite colonies were examined for the presence of seasonal variation in conception rate. Month of conception was inferred by counting back 9 months from date of birth. Over 4300 births, occurring between 1758 and 1964 and spanning the years that the colonies inhabited the Ukraine and then migrated to the United States and Canada, were included in the analysis. When combined, the European and North American births showed a seasonal pattern with a general rise in conceptions from December to June followed by a decline in conceptions from July to November. The major peaks in conceptions were in April and June, with a minor peak in December. Separate examination of the European and American records revealed a secular change. The seasonality of North American conceptions was dramatically reduced when compared to the very distinct European seasonal pattern. It is assumed that both biological and cultural factors are responsible for the seasonal variation observed. The influences of light cycle, date of marriage, and work and holiday schedules on conception rates are discussed.