Bacterial vaginosis is an overgrowth of bacteria — called Gardnerella vaginalis — that's normally found in the vagina. If you have it, you might notice a change in your vaginal discharge. It may become a milky-white or gray color, and it might smell strong or fishy.
It's not really clear whether pregnant women are more susceptible to bacterial vaginosis, but it's important to see your doctor if you think you might have it, since some studies have linked the condition with preterm birth. Luckily, it's easily treatable with pregnancy-safe antibiotics.