Abstract： Objective To examine the expression of tumor necrosis factor in placenta of pregnant rats with chronic periodontitis.Methods Twenty Wistar female rats were randomly divided into two groups,control(n=8)and experimentalgroup(n=12).The periodontitis model was established in the experimental group.The females and males in the two groups got together four weeks later.Nineteen days after pregnancy all rats were executed and placenta collected.The delivery time and neonatal birth weight were recorded and the pathological changes of periodontal tissue observed.Tumor necrosis factor(TNF)expression was examined in placenta by real-time quantitative polymerase chain reaction analysis.Results The animal model of chronic periodontitis was successfully established.Experimental group delivered 30 offspring and the control group 56 offspring.The average number of pups born alive per liner in experimental group(4.1±2.2)Was significantly lower than that in control group(9.2±2.2),P<0.05.The birth weight of pups in experimental group[(5.01±0.43)g]wag significantly lower than that in the control group[(5.79±0.53)g],P<0.05.The relative quantitative expression of TNF was(1.807±0.265)in experimental group the and (1.003±0.021)in the control group(P=0.001).Conclusions Chronic periodonfitis may be related to preterm low birth weight.