Changes in vaginal flora during pregnancy and association with preterm birth.

A prospective study of the vaginal flora changes between midtrimester and labor was undertaken in 560 women; 337 had intact membranes and were in labor when swabs were taken. Of these, 45 delivered preterm. Gardnerella vaginalis, Ureaplasma urealyticum (both associated with preterm birth when present in midtrimester), and Mycoplasma hominis commonly persisted between midtrimester and labor, particularly in the preterm cohort (24 [65%] of 37 midtrimester isolates), and these organisms were seldom acquired late in pregnancy. A second group, Bacteroides species, had both a high persistence (9 [60%] of 15 midtrimester isolates) and high late acquisition rate (10 [53%] of 19 labor isolates) in the preterm cohort. These findings provide a rationale for treatment of bacterial vaginosis in pregnancy. A third group, comprising enteropharyngeal bacteria and Peptostreptococcus species, were almost all acquired late in the preterm cohort (14 [88%] of 16 labor isolates), and persistence from midtrimester was rare.