An Analysis of the Second Birth Interval in Tehran, Iran: Trends and Correlates

Background & aim: Postponement of the second birth significantly affects the fertility rate. The main purpose of this paper is to estimate the survival function of the interval between first and second birth and its determinants. Methods: This study utilized a sub-sample of 363 ever-married women aged 15-49 with at least one child from the 2017 “Iran Fertility Transition Survey” conducted in Tehran and four other provinces, using a structured questionnaire for data collection. The Kaplan-Meier estimator was used to determine the second birth interval, while to analyse its determinants, the gamma-shared frailty distributions with the Weibull model were employed. Results: The median time from the birth of the first child to the second child was reported 84 months (The time ratio of the second birth interval for women with a diploma or less, compared to university graduate women, was 0.754 and 0.748, respectively. The time ratio of the second birth interval for married women in the 1980s. In comparison to recent marriage cohorts, was 0.651. Increasing the desired number of children resulted in a shorter interval between the first and second birth (TR = 0.786). Experience of abortion (TR = 1.23), prolonged working hours (TR = 1.010), and postponement of the first child (TR = 1.06) were linked to a longer interval between the first and second births. Conclusion: The second birth interval in Tehran exceeds the national average, influenced by urban residency, university education, and delays in the first childbirth.