Since periodontitis is a chronic oral infection which recently recognized as a potential risk factor for preterm birth, the interests to find the relationship between periodontal disease and adverse pregnant outcomes are raised. Preterm birth and low birth weight are world-wide leading perinatal problems, and their incidence rates are still high even in recent days. The present review focuses on the hypothesis how periodontitis can cause preterm birth, and to review the epidemiological studies including interventional studies, case-control studies, cohort studies, and systematic reviews. Either from the epidemiological studies, the microbiology, or the immunology findings, all suggested that periodontitis might be an important risk factor to preterm birth. Larger studies in different races with uniformed research design are recommended. Maintenance of good oral hygiene and adequate periodontal treatments are advised.