Objectives. We examined whether periodontal treatment or other dental care is associated with adverse birth outcomes within a medical and dental insurance database., Methods. In a retrospective cohort study, we examined the records of 23 441 women enrolled in a national insurance plan who delivered live births from singleton pregnancies in the United States between January 1, 2003, and September 30, 2006, for adverse birth outcomes on the basis of dental treatment received. We compared rates of low birthweight and preterm birth among 5 groups, specifying the relative timing and type of dental treatment received. We used logistic regression analysis to compare outcome rates across treatment groups while adjusting for duration of continuous dental coverage, maternal age, pregnancy complications, neighborhood-level income, and race/ethnicity., Results. Analyses showed that women who received preventive dental care had better birth outcomes than did those who received no treatment (P < .001). We observed no evidence of increased odds of adverse birth outcomes from dental or periodontal treatment., Conclusions. For women with medical and dental insurance, preventive care is associated with a lower incidence of adverse birth outcomes.