Periodontal diseases are a group of clinical entities, where periodontitis represents one of the most frequent disease nowadays. In the etiopathogenesis of these diseases, the key role is taken by microorganisms and by the establishment of their metabolism. The presence of periodontal pathogens in the organism is connected to many systemic conditions and diseases, and there are many studies that are aiming to discover the effects of periodontal pathogens on adverse pregnancy outcomes. Preterm delivery, low birth weight and pre-eclampsia are some of many conditions that can be connected to the presence of periodontal pathogens during pregnancy, such as the proinflammatory response, which is the result of an infection originating in these pathogens. Some of the above-mentioned adverse pregnancy outcomes may affect the foetus only, such as preterm delivery, meanwhile some may affect the mother, such as pre-eclampsia. It has been suggested that periodontal pathogens could reach the fetoplacental unit by and infection ascending from the birth canal or via hematogenous spread from distant infection focal points, such as periodontal pockets. These are the two potential mechanisms suggested as the explanation as to how periodontal pathogens may affect the adverse pregnancy outcomes. The complications, which are the result of the progression of the aforementioned conditions, may be considered as life-threatening conditions for the mother or for the foetus. The impact of periodontal diseases on pregnancy and their mutual relationship have been the main focus of many studies. The results of the studies vary, and further clinical trials are required.