There is growing evidence that chronic periodontitis may be a risk factor for pre-term birth. The goal of this intervention study was to determine the effect of periodontal treatment on the pregnancy outcome in women with threatening pre-term birth and initial localized chronic periodontitis. Forty-one women with a singleton pregnancy were enrolled in the study. For this treatment group, oral hygiene instruction and periodontal therapy were provided in the third trimester, while those in the control group (42 persons) did not receive any periodontal treatment. In the treatment group, the mean weight of newborns was 3079.0 g, compared with the control group mean of 2602.4 g. The incidence of pre-term birth and low birthweight in the treatment group was significantly less than in the control group (p = 0.015). Periodontal treatment completed before the 35th week appeared to have a beneficial effect on birth weight and time of delivery.