Background/Aim. Preterm delivery together with insufficient body weight and death cases in newborns is the main issue in obstetrics. About 40% of preterm delivery is caused by infections. The aim of this study was to investigate whether and which bacterial infections of genital tract can be associated with preterm delivery, and depending on when diagnosis was made. Method. The study involved 216 pregnant women. According to pregnancy outcome, two groups were formed. The study group involved 29 pregnant women who had preterm delivery out of which nine were examined in I trimester, eight in II trimester and 12 in III trimester. The control group involved 187 pregnant women out of which 47 were examined in I trimester, 73 in II trimester and 67 in III trimester. Bacteriological examination of vaginal and cervical swabs was done in all pregnant women. Infection was diagnosed by finding bacterial antigen in cervical swabs or positive cultures of vaginal and/or cervical swabs followed by the presence of the increased number of polymorphonuclears in direct microscopic preparation. Results. The results showed that in III trimester of pregnancy vaginal bacterial infection was statistically more common (p = 0.021) in women who had preterm delivery (66.7%) in relation to women who delivered in term (29.9%). In this period of gestation the increased number of polymorphonuclears in DMP of vaginal swabs is more common in the women of the study group (75%) than in the women of the control group (43.3%). Preterm delivery was registered in 16.1% women whose microbiological analyses were done in I trimester, 9.9% women in whom microbiological analyses were done in II trimester and in 15.2% pregnant women microbiologically tested in III trimester. Conclusion. Based on the obtained results it could be concluded that bacterial infections of genital tract and period of gestation when infection is diagnosed have influence on reducing perinatal morbidity and mortality caused by preterm delivery.