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To study the occurrence of breech presentation and its association with demographic and geographic variables.Population based cohort study from the Medical Birth Registry of Norway comprising all singleton deliveries 1967-1994, a total of 1,592,064 deliveries. Of these, 45,921 in breech presentation.From 1967 through 1994, the breech presentation proportion increased from 2.2% (95% CI 2.1-2.3) to 3.4% (95% CI 3.2-3.5). Breech presentation was associated with high maternal age and low birth order, as well as low gestational age and birthweight. The secular trend was mainly due to demographic changes in terms of increasing proportions of births with low birth order and high maternal age. Breech presentation was most frequent in urban areas.Strong associations were observed between breech presentation and low birth order as well as high maternal age. The findings are compatible with both intrinsic as well as environmental mechanisms. A full understanding of the birth order effect necessitates further studies based on sibship data. Prevention of premature delivery would be an effective measure for reducing the breech presentation proportion.