Background: Preterm labor is the leading cause of neonatal morbidity and mortality this study aimed to determine single and combined effects of all risk factors in relation to preterm labor. Methods: This was a case-control study that included patients from a teaching hospital affiliated to Tehran University of Medical Sciences. Women who came to this hospital for delivery were divided into two groups: 200 in preterm delivery (before 37 weeks) and 195 in term delivery (after 37 weeks). Results: In all 409 pregnant women were delivered. 217 were preterm (group A) and 192 were term (group B). The mean age of mothers in group A was 30.04 ± 5.74 and the mean age of mothers in group B was 27.28 ± 5.90 (P < 0.001). The risk of preterm labor was 4.22 higher in mother with any risk factor before or during pregnancy (P < 0.001), it was 3.67 higher in mother with complication of pregnancy (P < 0.001) and it was 3.40 higher in neonate with any complication (P = 0.002) compared to mother without any risk factors. For each mother, we calculated risk score for preterm labor by counting the number of risk factors. The risk of preterm labor was significantly higher in mother with risk factors compared to those without any risk factors (P for trend = 0. 002). Mother with two risk factors had a 5.60 (P = 0.01) and mother with three or more risk factors had a 23.48 (P = 0. 001) times higher risk for preterm labor than those who did not have any risk factors. Conclusion: The screening and identification of mothers with risk factors for preterm delivery can increasing mother's attention and participation to better manage of these condition to have more safe pregnancy period.