Background: Epidemiologic and randomized controlled studies have shown that periodontal diseases may be associated with preterm labor and delivery of infants with low birth weights. The purpose of the present study was to determine the presence of microbial invasion of the amniotic cavity by periodontopathic bacteria in pregnant women with a diagnosis of threatened premature labor. Methods: A periodontal examination and collection of amniotic fluid and subgingival plaque samples were performed on women identified as having threatened premature labor (preterm premature rupture of membranes without clinical infection or labor and preterm labor with intact membranes) and a gestational age ranging between 24 and 34 weeks. Samples collected from amniotic fluid and from the four deepest periodontal pockets in each patient were pooled in prereduced transport fluid and cultured. Porphyromonas gingivalis was identified primarily by colony morphology under stereoscopic microscope and rapid biochemical tests. Amniotic fluid or plaque samples were homogenized, DNA was extracted, and polymerase chain reaction (PCR) amplification of 16S rRNA with specific and universal primers was carried out. Results: Twenty-six women with threatened premature labor were included: eight with preterm premature rupture of membranes and 18 with preterm labor with intact membranes. Eight women presented with gingivitis, 12 with chronic periodontitis, and six without periodontal disease. Microbial invasion of the amniotic cavity as detected by P. gingivalis PCR was 30.8% (eight of 26 patients). In these eight patients, P. gingivalis was present in both the subgingival samples and the respective amniotic fluid sample. Conclusion: The presence of microbial invasion of the amniotic cavity by P. gingivalis could indicate a role for periodontal pathogenic bacteria in pregnant women with a diagnosis of threatened premature labor.