A randomized controlled trial is being conducted with recruitment of women from the antenatal clinics of Perth’s major obstetric centres and associated private practices. The sample is being enriched by inviting women with a history of poor oral health or pre-term birth. 5400 eligible women will undergo a hygienist-based screening study at 12-20 weeks gestation. 1094 women who meet the criteria for periodontal disease determined from our recent pilot study will be allocated at random to the treatment or control groups. Those in the treatment group will receive three one-hour hygienist-based periodontal treatment sessions commencing at 20 weeks gestation, followed by re-examination at 26 weeks. Re-treatment will be offered to the 5% of cases in which treatment has been ineffective, and ongoing maintenance to the remainder. Women allocated to the control group will be offered similar treatment after completion of the pregnancy.