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An improved understanding of the animal factors that affect measures such as conception rate would contribute to solving the problem of impaired reproductive performance in modern dairy cows. A question of particular interest relates to the observed improvement in conception rates from first to second and third oestrus cycle: is the increase in conception rate related to cycle number per se or to increasing days from calving? A 5-year study using three breeds (Holstein, Jersey and Danish Red) allowed this issue to be examined. In 560 lactations, from calving until confirmed pregnancy or until 290 days after calving all cows were observed daily for signs of oestrus. Inseminations were carried out to all clear cases of oestrus after 35 days from calving until the cow was judged to be pregnant or until 290 days post-calving. Days from calving to the first oestrus was modeled using a frailty model. Danish Red cows had a significantly greater rate of occurrence of first oestrus over time. Generalized linear mixed models defined using a binomial distribution and logit link function were used to estimate probability of pregnancy as affected by: breed, parity, oestrus number and days from calving to first oestrus. In order to account for the paradox that the incidence of cows with inherently low fertility increases with oestrus number, different random components for cow were included. There was a significant increase in probability of pregnancy from first to second oestrus but no significant change in probability of pregnancy thereafter. There was a significant increase in probability of pregnancy with increasing days to first oestrus. This indicates that both oestrus number and days from calving play a role in determining the fertility of dairy cows. Correcting for inherent differences in fertility was shown to be important Udgivelsesdato: April An improved understanding of the animal factors that affect measures such as conception rate would contribute to solving the problem of impaired reproductive performance in modern dairy cows. A question of particular interest relates to the observed improvement in conception rates from first to second and third oestrus cycle: is the increase in conception rate related to cycle number per se or to increasing days from calving? A 5-year study using three breeds (Holstein, Jersey and Danish Red) allowed this issue to be examined. In 560 lactations, from calving until confirmed pregnancy or until 290 days after calving all cows were observed daily for signs of oestrus. Inseminations were carried out to all clear cases of oestrus after 35 days from calving until the cow was judged to be pregnant or until 290 days post-calving. Days from calving to the first oestrus was modeled using a frailty model. Danish Red cows had a significantly greater rate of occurrence of first oestrus over time. Generalized linear mixed models defined using a binomial distribution and logit link function were used to estimate probability of pregnancy as affected by: breed, parity, oestrus number and days from calving to first oestrus. In order to account for the paradox that the incidence of cows with inherently low fertility increases with oestrus number, different random components for cow were included. There was a significant increase in probability of pregnancy from first to second oestrus but no significant change in probability of pregnancy thereafter. There was a significant increase in probability of pregnancy with increasing days to first oestrus. This indicates that both oestrus number and days from calving play a role in determining the fertility of dairy cows. Correcting for inherent differences in fertility was shown to be important