Tempo of fertility is the average waiting time to next birth. Fertility as an important determinant of population dynamism has a special study concern in socio-economic factors. We aimed to determine the effects of some factors on inter-birth time spacing by Cox regression model and calculate the tempo of fertility.

 Through a cross-sectional study in 1384, 978 women in fertility age from rural areas of Zarrindasht (situated on the east of Fars province) were selected by multi-stage random sampling. The birth of each child was considered as the event. The impact of some socio-demographic factos on fertility was assessed by Cox proportional hazard model. The important effective variables on birth spacing were marital age of mother, mother age at index birth, breast feeding duration and mother education. The results are similar to other studies.