Data from epidemiological and clinical studies published in the past two decades indicate certain association between periodontal disease and increased risk for preterm birth or low birth weight. Although the strength of those observed associations is weak, periodontitis today is considered as one of the potentially modifiable risk factors for adverse pregnancy outcomes. The aims of the publication are to summarize the epidemiological and clinical evidence for the impact of periodontal disease on adverse pregnancy outcomes and to make an attempt to overview the potential biological mechanism behind this association. The majority of epidemiological and clinical studies found certain negative effect of poor maternal periodontal condition on the incidence of low birth weight, preterm birth, pre-eclampsy, restricted foetal growth or even stillbirth. Two possible biological pathways have so far been identified: 1) the direct dissemination of the periodontal pathogens or their toxic by-products which reach the foetal-placental unit, and 2) an indirect mechanism when the circulating systemic inflammatory mediators induced by the periodontal inflammation can provoke secondary inflammation and foetal damage in the amnion. The periodontal therapy applied during the second or third trimesters has not been proven to reduce the incidence of any adverse pregnancy outcomes in pregnant women. A much more prophylaxis-oriented approach in periodontal treatment is needed. The adequate periodontal therapy should be completed before the conception to provide benefit to the pregnant women and also the new born baby. Orv Hetil. 2018; 159(25): 999-1007.