Fertilities Acceleration (Tempo) and Factors Affecting the First Birth Interval in Iranian Women: A Survival Analysis

Background: A short pregnancy interval (long fertility acceleration (Tempo)) is associated with adverse health and perinatal outcomes, including maternal, child, and neonatal mortality. Objectives: We aimed to investigate the duration and determining factors of first birth intervals among women of reproductive age in Semnan, Iran. Methods: In this cross-sectional study, the fertility history of 600 married women aged 15 to 49 in Semnan (Iran) was investigated from the list of health centers. Individuals were selected using a multistage sampling method in 2018. Finally, a married woman (15 - 49 years) was selected randomly from each household. Data were obtained by interview questionnaire, calculated tempos, and analyzed with a Cox proportional hazard model. Results: The median (first – third quartile) time of first birth intervals and age of women and men at the first marriage were estimated to be 24.0 (12 - 44) months, 20 (18 - 23), and 25 (22 - 28) years, respectively. The lowest estimated tempo was for third births (68.63 months), and the highest was for the 1st (26 months). Among explanatory variables of interest, the number of children expected, the father's age at the first marriage, the mother's occupation, and the desire to have children were significant predictors of the first birth intervals. Conclusions: Due to the decrease in the number of births in the past several years and also the prediction of a decrease in the coming years, one of the ways to reduce the birth interval and, as a result, increase the fertility rate is to reduce the age of marriage among young people and in general to marry on time among young people, as well as to control inflation and reduce the cost of having children.