The preterm delivery of low-birth-weight (PLBW) infants remains a significant public health issue and a leading cause of neonatal death and of long-term neurodevelopmental disturbances and health problems. Epidemiological and immunological studies have suggested that periodontal disease may be an independent risk factor for PLBW. The incidence of periodontal disease during pregnancy has been found to be associated with a significantly lower birth weight for gestational age after adjustment for race, parity and baby gender. The present case-control study, carried out to examine the distribution of anaerobes in pregnant women with periodontitis who experienced preterm delivery, provides microbial evidence that maternal periodontal disease and the presence of key pathogens are significant contributors to the obstetric risk of preterm delivery.