Background and Aim: Fertility preferences and first birth interval are two effective components on low fertility, which are emphasis by planners and politicians. The aim of this study is to evaluate factors affect on ideal first birth interval among pre-marriage youths.

Materials and Methods: A sample of 12500 pre-marriage youth women and men in all provinces who visited pre-marriage counseling centers in 2014, based on cross-sectional survey by multistage stratified sampling method and structured questionnaire was gathered. In this article, ideal first birth interval as a response variable was modeled on gender, place of residence, age, educational level, ideal number of children, job status, family income level and sex preference by parametric survival analysis.

Results: Since in this study proportional hazard assumption for Cox model was not met, parametric survival analysis was used. In all provinces, log-normal model between exponential, Weibul and log-logistic was selected and interpreted based on AIC. Age and job status, in all provinces, affected ideal first birth interval significantly (α=0.05). Gender, ideal number of children, family income level and sex preference also affected on this interval in category 1 to 3 of province.

‍Conclusion: In all provinces, the ideal first birth interval decreases by increasing age. This may be due to fear of infertility by delaying childbearing is the cause of this result. Also, university students have longer ideal first birth interval than the other job status levels. Therefore, it seems youth interesting for education, will delay childbearing more than past.