This was a prospective cohort study. COHORT SELECTION: The initial sample comprised 1404 pregnant women seeking prenatal care at the University Hospital of Lleida (Spain) between March 2003 and January 2005.Periodontal data [full-mouth data, pocket probing depths (PPD) and clinical attachment levels (CAL) and bleeding on probing], pregnancy outcome variables and information on other factors that may influence adverse pregnancy outcomes were collected. Data were analysed using a logistic regression model.The incidence of preterm birth (PB) and low birthweight (LBW) was 6.6% and 6.0%, respectively. The incidence of preterm LBW (PLBW) was 3.3%. PB was related to mother's age, systemic disease, onset of prenatal care, previous PB, complications of pregnancy, type of delivery, the presence of untreated caries and the presence of periodontitis (odds ratio, 1.77; 95% confidence interval, 1.08-2.88). LBW was related to the mother's smoking habits, ethnicity, systemic diseases, previous LBW babies, complications of pregnancy and type of delivery. PLBW was related to the mother's age, onset of prenatal care, systemic diseases, previous LBW babies, complications of pregnancy and type of delivery.The factors involved in many cases of adverse pregnancy outcomes have still not being identified, although systemic infections may play a role. This study found a modest association between periodontitis and PB. Further research is required to establish whether periodontitis is a risk factor for PB and/ or LBW.