Lily Nichols highlights this very important research brief:
In one well-designed randomized trial, 99 pregnant women who tested positive for GBS at 35-37 weeks were given either a probiotic supplement or placebo. The probiotic supplement contained 10 billion CFU (each) or two strains of probiotic bacteria that have been specifically studied for their beneficial effects on vaginal health: Lactobacillus rhamnosus, GR-1® and Lactobacillus reuteri RC-14®.
The probiotic supplement was taken for an average of 20 days. At the time of admission to the hospital for delivery, a repeat screen for GBS was completed. Among the women who received probiotics, 43% tested negative for GBS compared to only 18% in the placebo group. Those are pretty compelling statistics, though note that it did not reverse colonization in all cases.
The study authors propose that supplementation w/ these strains be started in the third trimester.