Childbearing: The First Birth Interval and Its Determinants in the Employees of Tehran University of Medical Sciences, Iran, 2018 - 2021

Background: Childbearing is a major concern in some countries around the world. The fertility pattern can be described by several indicators, such as the first birth interval (FBI). Objectives: This study aimed to predict the factors affecting the time of first birth. Methods: This cross-sectional study is part of a cohort study in Iran (2018-2021). The participants were 986 married women who were employees at the Tehran University of Medical Sciences (TUMS) and enrolled by the census method. A checklist was used to collect information. Data were described by mean, SD, frequency, and the effect of the variables on first birth and analyzed by the Cox analysis. Results: The adjusted hazard ratio (aHR) of the first birth was higher in the women with a high level of education than in those with a lower level of education (aHR = 0.29; 95% CI, 0.16 - 0.52; P < 0.001). Also, the hazard of earlier first birth tripled in women with higher occupation levels than those with lower occupation levels (aHR = 0.34; 95% CI, 0.59 - 0.96; P < 0.05). The hazard of first birth was lower in married women under 20 years than in those with other cohort age groups (aHR = 1.87; 95% CI, 1.09 - 1.89; P < 0.001). Conclusions: The age at first birth is significantly under the effect of women's education, occupation, marriage age, and wealth index.