Studies on the link between periodontal disease and adverse pregnancy outcome have gone through several phases. The epidemiological studies predominantly support a positive association between these wide-affecting diseases. During the intervention phase, a few small-scale, single-center studies reported improvement of birth outcome following periodontal treatment, whereas the large-scale multi-center studies did not demonstrate efficacy. Many questions arise with regard to patient population, disease type, and therapy. In addressing these questions, it is crucial that one understands the mechanism underlying the link between these diseases. Two non-mutually exclusive hypotheses exist. In the first, periodontal disease is believed to affect the maternal and fetal immune responses systemically, leading to premature labor. Alternatively, evidence is accumulating that oral bacteria may translocate directly into the pregnant uterus, causing localized inflammation and adverse pregnancy outcome in the presence or absence of clinical periodontitis. The oral-uterine transmission is not limited to the well-recognized periodontal pathogens, but instead may also involve the commensal species. Future studies should investigate these mechanisms, to understand the host susceptibility to oral-uterine transmission. Only when a thorough understanding of the mechanism is achieved can meaningful intervention studies be designed utilizing effective therapies, targeting appropriate populations, and measuring relevant outcomes.