Determinants of Birth Intervals Using PWP-Gap Time Model: Tehran Case Study

Background: Total fertility rate (TFR) in Iran decreased from the year 2000 and recently Iran has experienced fertility rates below replacement level. Birth interval is one of the most important determinants of fertility levels and plays a vital role in population growth rate. Due to the importance of this subject, the aim of this study was analyzing three birth intervals using three Survival Recurrent Event (SRE) models. Materials and Methods: In a 2017 cross-sectional fertility survey in Tehran, 610 married women, age 15-49 years, were selected by multi-stage stratified random sampling and interviewed using a structured questionnaire. The effects of selected covariates on first, second and third birth intervals were fitted to the data using the Prentice-Williams- Peterson-Gap Time (PWP-GT) SRE model in SAS 9.4. Results: Calendar-period had a significant effect on all three birth intervals (p