Prevalence of vaginal microorganisms among pregnant women according to trimester and association with preterm birth

Objective The aim of this study was to investigate the prevalence of abnormal vaginal microorganisms in pregnant women according to trimester, and to determine whether the presence of abnormal vaginal colonization is associated with higher risk of miscarriage or preterm delivery. Furthermore, we analyzed delivery outcomes according to individual microorganism species. Methods We included pregnant women who underwent vaginal culture during routine prenatal check-up between January 2011 and June 2016. We compared delivery outcomes according to the presence or absence of abnormal vaginal flora grouped by trimester. Results This study included 593 singleton pregnancies. We classified participants into 3 groups, according to the trimester in which vaginal culture was performed; 1st trimester (n=221), 2nd trimester (n=138), and 3rd trimester (n=234). Abnormal vaginal colonization rate significantly decreased with advancing trimester of pregnancy (21.7% for 1st, 21.0% for 2nd, 14.5% for 3rd; P=0.048). Abnormal vaginal colonization detected in the 2nd trimester but not in 1st trimester was associated with a significant increase in preterm delivery before 28 weeks of gestation (6.9% vs. 0%; P=0.006). Among abnormal vaginal flora isolated in the 2nd trimester, the presence of Klebsiella pneumonia was identified as significant microorganism associated with preterm delivery before 28 weeks of gestation (50% vs. 0.7% for K. pneumonia; P=0.029). Conclusion There is an association between abnormal vaginal colonization detected in the 2nd trimester and preterm delivery before 28 weeks. K. pneumonia has been identified as the likely causative microorganisms.