Objective To investigate the associations between periodontal diseases, presence of Porphyromonas gingivalis（P.gingivalis）, serum levels of pro-inflammatory mediators and preterm low birth weight（PLBW）. Methods 60 women（30 PLBW and 30 healthy women）, were recruited after postpartum within 3 days in this case-control study. Periodontal measurements including plaque index（PI）, bleeding index（BI）, probing pocket depth（PD） and clinical attachment loss（CAL） were assessed. The subgingival plaque was collected before periodontal examination and analysed by polymerase chain reaction（PCR） for the presence of the 16S rRNA gene specific to P.gingivalis, while the venous and umbilical cord blood specimens collected were determined by enzyme-linked immunosorbent assay（ELISA）. Results The PLBW cases had a poorer oral conditions and the presence of P.gingivalis was found in a higher proportion in the PLBW than the healthy pregnant（56.7% vs. 30.0%, P
