Of 322 pregnant women with periodontal disease, 160 were randomly assigned to receive scaling and root planing (SRP, cleaning above and below the gum line), plus oral hygiene instruction, whereas the remaining 162 received only oral hygiene instruction and served as an untreated control group. Subjects received periodontal examinations before and 20 weeks after SRP, and were classified blindly according to the results of treatment into two groups: successful (‘non-exposure’) and unsuccessful (‘exposure’) treatment. Groups were compared using standard inferential statistics; dichotomous variables were compared using the chi-square test or logistic regression. Results are presented in terms of odds ratios.