Background: Periodontal infections are the most common oral diseases in pregnancy. This condition can cause complications during pregnancy. Therefore, this study was designed to determine the factors affecting preterm birth and their relationship with periodontal diseases. Methods: This was a prospective cohort study and the population, consisted of women 18-45 years attending prenatal care centers in their second trimester of pregnancy. After obtaining informed and free consent from the female parent, a periodontal examination was performed and her medical information was recorded. Examination of Rumford’s teeth was performed to assess the severity of gingivitis, plaque, calculus, and attachment loss, and AMOS software was used to investigate the association between preterm labor and delivery and to provide a conceptual model. Results: The consequences showed a direct association between preterm labor and factors, such as attachment loss, calculus , bleeding on probing (BOP), age, current diabetes, gestational diabetes, gestational hypertension, lack of Calcium, Folic acid, and Fe supplementation, mothers’ depression, polyhy dramnios, smoking severity and history of premature rupture of the membrane. The results also indicated an indirect relationship between an irregular tooth brushing and preterm birth. Conclusion: Attachment loss, calculus, bleeding on probing are directly related to premature labor and irregular tooth brushing is indirectly associated with early birth. Due to the direct relationship between periodontal diseases and preterm delivery, regular brushing, oral hygiene, oral examinations during pregnancy and follow-up treatment of periodontal disease during pregnancy are recommended. Keywords: Periodontal disease, Preterm labor, Cohort study, Path analysis, Pregnancy