There were increasing studies on the association of periodontitis with preterm low birth weight (PLBW) in the recent 10 years. PLBW is associated with about half of all perinatal mortality. Known factors which could induce PLBW include smoking, genetics, alcohol, prenatal care, nutrition, urinary tract infections, However, there are still about 25% of PLBW cases occur with etiology unknown. Many evidences supported the association between periodontitis and PLBW. Periodontitis is a risk factor of PLBW, but the mechanism is unclear. Current theory suggests that periodontal inflammation adversely affects pregnancy outcomes by one or two mechanisms. First, women with periodontal disease may experience more frequent and severe bacteremia than periodontally healthy women. As a result, the uterine cavity may become exposed to or colonized by periodontal bacteria or their byproducts (e.g. lipopolysaccharides). Once they reach the maternal-fetal unit, oral bacteria may elicit an inflammatory cascade that leads to preterm labor. A second putative mechanism does not require oral bacteria to colonize the uterine cavity. Rather, cytokines generated within the diseased periodontal tissue may enter the systemic circulation and precipitate a similar cascade, again leading to spontaneous preterm labor and birth. Therefore, oral health instruction and periodontal treatment may decrease the infection of periodontal pathogens and reduce the risk of PLBW. For the present, the best advice for a woman who contemplate pregnancy is effective brushing for two times per day and regular periodontal treatment.