Background  Recent studies suggest that periodontal disease, as a source of subclinical and persistent infection, may induce systemic inflammatory responses that increase the risk of adverse pregnancy outcomes. Objectives  To examine the existing evidence on the relationship between periodontal disease and adverse pregnancy outcomes. Search strategy  Published studies identified via searches of the MEDLINE, EMBASE, CINAHL, and Current Contents full‐text databases. Selection criteria  We identified and selected observational studies (i.e. case–control, cross‐sectional, and cohort) and nonrandomised controlled studies or randomised controlled trials that examined periodontal disease as a risk factor for adverse pregnancy outcomes. Data collection and analysis  Odds ratios (OR) or risk ratios (RR) were extracted or calculated from the studies’ data. We calculated pooled effect size for two clinical controlled trials but not for the observational studies due to the heterogeneity in definitions for periodontal disease and adverse pregnancy outcomes across studies. Main results  Twenty‐five studies (13 case–control, 9 cohort, and 3 controlled trials) were identified. The studies focused on preterm low birthweight, low birthweight, preterm birth, birthweight by gestational age, miscarriage or pregnancy loss, and pre‐eclampsia. Of the chosen studies, 18 suggested an association between periodontal disease and increased risk of adverse pregnancy outcome (ORs ranging from 1.10 to 20.0) and 7 found no evidence of an association (ORs ranging from 0.78 to 2.54). Three clinical trial studies suggest that oral prophylaxis and periodontal treatment can lead to a 57% reduction in preterm low birthweight (pooled RR 0.43; 95% CI 0.24–0.78) and a 50% reduction in preterm births (RR 0.5; 95% CI 0.20–1.30). Author's conclusions  Periodontal disease may be associated with an increased risk of adverse pregnancy outcome. However, more methodologically rigorous studies are needed for confirmation.