Factors Affecting First and Second Birth Intervals Among 15-49 Year-Old Women in Tehran

One of the most important determinants of the fertility level is the birth interval. Considering the importance of this issue, the aim of this study was to analyze the first and second birth intervals using shared frailty survival model and comparing factors affecting these intervals.
  Probability proportional to size stratified sampling was used to select 610 married women aged 15-49 years from different regions of Tehran during the winter and spring of 2017. Data were collected using a structured questionnaire. The shared frailty survival model was fitted to investigate the effect of calendar period, age at marriage, education level, education level of the spouse, job and migration status, household costs, and place of residence on first and second birth intervals.
  The median length of the first and second birth interval was 38 and 55 months, respectively. Calendar period had a significant effect on the first birth interval (p-value=0.016). Job status (p-value=0.045) and place of residence (p-value=0.025) had a significant effect on the second birth interval. The hazard rate  of the first birth interval for women in the recent calendar period compared to women in the first period was equal to 0.448, and the hazard rate of the second birth interval for employed compared to unemployed, living in developed versus undeveloped regions was 0.812, and 0.724, respectively
  Delayed childbearing among young women and longer second birth intervals in employed women may result from economic and social conditions that can be prevented by providing appropriate conditions.