Tehran University of Medical Sciences Journal of Family and Reproductive Health 1735-8949 14 1 2020 07 18 52 56 Maryam Asgharnia Reproductive Health Research Center, Guilan University of Medical Sciences, Rasht, Iran Tahereh Varasteh Reproductive Health Research Center, Guilan University of Medical Sciences, Rasht, Iran Davoud Pourmarzi School of Public Health, Faculty of Medicine, The University of Queensland, Brisbane, Australia 2019 12 15 2020 03 09 Objective: Preterm birth is associated with high rates of neonatal morbidity and mortality. This study aimed to investigate the relationship between inter-pregnancy interval and the incidence of preterm birth. Materials and methods: In a case-control study, 185 women with preterm delivery and 185 women with term delivery were included. Data including inter-pregnancy interval, demographic characteristics, history of prenatal and neonatal complications, parity, gravidity, type of delivery, and smoking status were collected. Results: The mean of the inter-pregnancy interval in the case and control groups were 79.84 ± 45.55 months and 78.49 ± 41.29 months, respectively (P = 0.767). Inter-pregnancy interval 12-month or less in comparison with Inter-pregnancy interval more than 24 months significantly increased the odds of preterm delivery (OR: 4.05, 95% CI: 1.06-15.39, p = 0.040). However, inter-pregnancy interval of 13-24 months was not a risk factor when compared with more than 24-month inter-pregnancy interval (OR: 1.54, 95% CI: 0.62-3.80, p = 0.351). Having an educational level less than high school in comparison with tertiary level decreased the odds of preterm delivery (OR: 0.25, 95% CI: 0.11-0.56, P = 0.040). With each increase in number of gravidity odds of preterm delivery increased by 1.5 times (95% CI: 1.11-2.04, P = 0.009). Having a history of preterm delivery (OR: 2.57, 95% CI: 1.17-5.64, P = 0.019) and experiencing preeclampsia (OR: 1.98, 95% CI: 1.06-3.68, P = 0.032) increased the odds of preterm delivery. Conclusion: Inter-pregnancy interval of 12-month or less in comparison with more than 2-year inter-pregnancy interval, experiencing preeclampsia, history of preterm delivery and increased number of gravidity increase the risk of preterm delivery. Health care providers need to be informed with the appropriate inter-pregnancy interval and counsel women to make an informed decision regarding their pregnancy. https://jfrh.tums.ac.ir/index.php/jfrh/article/view/1272 https://jfrh.tums.ac.ir/index.php/jfrh/article/download/1272/513