The diploma thesis focused on the analysis of selected factors affecting the length of gestation period of cows. The observation was performed on 6 836 individuals. Results showed that duration of pregnancy was influenced by parity, sex of born calves and interaction between month of conception and parity. Duration of pregnancy was also influenced by month and year of born calves and sex of singletons and twins. Likewise, the duration of pregnancy was influenced by interaction between the month of conception of cows and the sex of singletons, alive and stillborn calves and parity. In conclusion, the results suggest that the duration of gravidity is affected by many factors which have not been fully clarified yet.