Objective (s): First birth interval has received special attention in public health and demographic investigations due to its implication in fertility, and maternal and child health. Study of ideal first birth interval also is a very important issue since it demonstrates people's attitude towards fertility. The aim of this study was to investigate the factors, which affect ideal first birth interval among pre- marriage youths. Methods: A sample of 12500 pre-marriage male and female youths in

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... provinces of Iran who visited pre-marriage counseling centers was selected. The sample selection was based on multistage stratified sampling method. Data were collected in year 2014 using a structured questionnaire. The ideal first birth interval as outcome variable was indicated by univariate survival analysis and stratified Cox model while controlling for some independent covariates. Results: Based on univariate survival analysis, age, ideal numbers of children and employment status were found to have significant effects on the mean of ideal first birth interval in all provinces. However by fitting different stratified Cox models to different groups of provinces, different covariates were found to play significant effect. Conclusion: Although there were different factors for influencing ideal first birth interval in different provinces, the differences were in the same direction. In all provinces, ideal first birth interval decreased by increase in age and university students favored longer ideal first birth interval than other participants. Risk of early childbearing for women, people who lived in rural area, had lower educational levels, had greater ideal number of children and had sex preferences were higher compared to the other groups.