A sample size of 540 women in each arm provides power to detect a reduction in the rate of pre-term birth from 12% to 7% with power of 90% and an alpha level of 5%. Our pilot study of 272 women revealed a rate of periodontal disease of 15% in unselected women. With enrichment by advertising for women at higher risk, we estimate the rate of periodontal disease will be 20%. Thus, for a total study size of 1080 women, 5400 need to be screened, which is rate of 45 each week in a 48 week working year. This sample size estimate allows two interim analyses.