Backgrounds: Abnormal vaginal discharge is a common problem among pregnant women. The most common cause of these discharges is bacterial vaginosis (BV), which has numerous complications and causes problems for pregnant mothers and their fetuses. The purpose of this study was to determine the BV frequency among pregnant women referring to a gynecology clinic in Arak city using Amsel and Nugent criteria, Alberta guideline, and PCR. Materials & Methods: This descriptive study was performed on 70 vaginal samples of pregnant women in Arak to investigate the most common causes of vaginal discharge according to Amsel and Nugent criteria and polymerase chain reaction (PCR) method using specific primers targeted towards three bacteria: Gardnerella vaginalis, Atopobium vaginae, Mobiluncus curtisii. Data were analyzed using SPSS software and Chi-square test. Findings: In this study, ten (14.28%) out of 70 pregnant women had positive bacterial vaginosis according to Amsel criteria. According to Nugent criteria and Alberta guideline, three (4.29%) cases were diagnosed with definite BV, 20 (32.26%) cases with intermediate BV with clue cells, 42 (67.74%) cases with intermediate BV without clue cells, and finally five (4.29%) cases with negative BV. Also, according to PCR, the frequency of G. vaginalis, M. curtisii, and A. vaginae in vaginal samples was 71.42% (50 cases), 64.28% (45 cases), and 30% (21 cases), respectively. Conclusion: According to the obtained results, the prevalence of definite bacterial vaginosis was lower than that of vaginitis, and most patients suffered from nonspecific vaginitis.