The waiting time between children births are called tempo. This phenomenon plays a crucial role in the
child and mother health. The purpose of the present study is determination of factors which influence this
variable of interest. A significance level of 5% and power of 80% considered to calculate the required
sample size for this cross sectional study. The sample size of 124 women determined using PASS software
(ver. 11.0.4). These women randomly selected of married women between 15-45 years old with at least two
children, living in Irin village. We consider the waiting time between the first and the second birth in women
randomly sampled from Irin village of Tehran province of Iran.The selected mothers had 364 children at the
time of study. The average and standard error for the number of children were 2.940 and 1.102 respectively.
59% of the children were girl and the rest were boy.Tempo variable as an index estimated using Toki
method. The lowest tempo is for 5th to 6th births (28.5) and highest is for 6th to 7th (60). Cox regression
model was used to determine the significant explanatory factors.Birth of child was considered as an event
and time between the first and the second event was considered as outcome in this model.According to the
fitted Cox regression model, only maternal education and father’s occupation were statistically significant at
5% on time to second birth.