Treating periodontal infections during pregnancy is unlikely to reduce the risk of preterm birth. A trial of dental hygiene treatment was conducted among pregnant women in Pennsylvania. The initial phase was the development of standard criteria to determine if periodontal disease was present. This involved measurement of gingival pocket depth and connective tissue attachment to teeth. In the second phase, 322 pregnant women with periodontal disease were given oral hygiene instruction and free supplies of floss, brushes and toothpaste. Half were randomised to have dental scaling and root planing as well. No significant difference in the incidence of preterm births was seen between the two groups; 52.4% in the control group and 45.6% in the intervention group. The third phase of the study looked at those women who had the scaling and root planing. All received treatment before 20 weeks gestation and an assessment of the success or otherwise of treatment 20 weeks later. Success equated to improvement in periodontal disease criteria or being no worse. A total of 49 successful and 111 unsuccessful cases were identified. The numbers of preterm deliveries were 4 and 69 respectively, indicating a strong association between periodontal disease progression in pregnancy and preterm delivery. There was a significant relationship between successful periodontal treatment and full-term birth.