In last decade, pregnancy loss in dairy cattle has had an upward trend bringing difficulties for breeders: the annual cost is estimated around 396 billion Rials (i.e. around 40 million US$) for the Iranian dairy industry. The present study was conducted to determine the influence of maternal factors on abortion and to predict the probability of abortion as well as the effect of these factors on the fetal lifetime in Holstein dairy cattle. Data from 44,629 established pregnancies that included 14,226 heifers and 30,403 pregnancies from 12,265 parous cows in nine industrial dairy herds around Tehran were used. Overall, 4871 pregnancies of parous cows resulted in abortion. Prediction of the probability of abortion (PPA) was estimated by a logistic regression model. Survival analysis was performed using an accelerated failure time (AFT) model assuming a multi-modal hazard function. Effective factors included age of dam at conception, gravidity, open days, number of previous abortion(s), abortion before/after 60 days of gestation in previous conception, herd and season of insemination. The PPA decreased with increasing open days, increasing gravidity and no previous abortion. In addition, the PPA was greater for cows which had been inseminated during summer versus winter. However, the difference between autumn and spring was not significant. Overall, 25 sires out of 695 from which sperm was collected for artificial insemination (AI) had significantly higher risk of abortion, with odds ratios ranging between 1.44 and 4.73 compared to the average. The survival probability increased slightly during gestation as gravidity increased for cows that had a previous abortion. Cows that had aborted before 60 days of gestation in previous conception tended to abort later in their next conceptions.
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