Remarkable changes in the life course of individuals have occurred in Iran during the past few decades. These changes are mirrored in reducing fertility rates, increasing the average age at marriage, first birth, and the reduction of early childbearing. Previous studies focused on fertility transition and birth interval in Iran. Using the 2000 Iran Demographic and Health Survey (IDHS), this paper applies a survival analysis model to estimate the interval between marriage and first birth. Determinants associated with the risk of first birth are also examined. The result shows that the median length of the first-birth interval is 2.7 years and more than 85 percent of women progress to first birth within 5 years of their marriage. Time trends indicate a delay of the first birth from the 1990 or later marriage cohorts in Iran, and this postponement has strong associations with marriage age and recent tendencies of women for higher education. Living in less-developed provinces can also be associated with a higher probability of having the first birth within two years of marriage. However, this probability reduces in the third year of marriage as compared with that of other provinces.