This was a randomised controlled trial (RCT). Participants were assigned randomly to receive periodontal treatment either before 23 weeks' gestational age or after delivery. Treatment consisted of up to four sessions of supragingival and subgingival scaling and root planing using hand and ultrasonic instruments to complete the baseline therapy. Local anaesthesia was used as needed. Those in the treatment group also received full-mouth tooth polishing and oral hygiene home instructions. There were no followup periodontal treatment visits. After delivery, those in the delayed-treatment group received periodontal therapy at no cost. The principal outcome (gestational age <37 weeks) included induced or spontaneous deliveries, foetal demise and miscarriage, but not therapeutic abortions. Secondary outcomes included gestational age <35 weeks; mean birth weight among neonates adjusted for race, sex and gestational age; and a composite measure of neonatal morbidity and mortality before discharge. The trial randomised 1806 patients at three performance sites and 1760 evaluable patients completed the study. At baseline, there were no differences between the treatment and control arms for any of the periodontal or obstetric measures. The rate of preterm delivery for the treatment group was 13.1%, and 11.5% for the control group (P 0.316). There were no significant differences when comparing women in the treatment group with those in the control group with regard to the adverse event rate or the major obstetric and neonatal outcomes. Periodontal therapy did not reduce the incidence of preterm delivery.