Iran has experienced one of the most rapid falls in fertility ever recorded. The observed pattern is one in which fertility has fallen simultaneously in all age groups, in all geographic settings and in all social groups, hence accounting for the rapidity of the decline at the national level. Available single calendar year time trends in the age-based total fertility rate and in age specific fertility rates elicit an interpretation of the fertility decline that is dominated by the influence of cross-sectional events such as the Islamic Revolution and the re-establishment of the family planning programme. In this paper, we show that an analysis based on synthetic parity progression ratios provides a much richer interpretation of the trend in period fertility in Iran than does the conventional age-based model. In particular, the parity-duration model better identifies the impact on cross-sectional fertility of changes in the timing of marriage and successive births.