Bacterial vaginosis is the most common cause of abnormal vaginal odor and discharge. It is caused by a change in the type of bacteria found in the vagina. Normally, bacteria belonging mostly to the Lactobacillus family live harmlessly in the vagina and produce chemicals that keep the vagina mildly acidic. In bacterial vaginosis, Lactobacillus bacteria are replaced by other types of bacteria that normally are present in smaller concentrations in the vagina.
Scientists do not fully understand the reason for this change. Risk factors that seem to increase the likelihood of bacterial vaginosis include a history of multiple sex partners, a sexual relationship with a new partner, cigarette smoking, vaginal douching and the use of the intrauterine contraceptive device (IUD). Although most of these risk factors are related to sexual activity, women who have never had vaginal intercourse can also develop bacterial vaginosis.
Bacterial vaginosis often occurs during pregnancy. It may cause premature labor and delivery, premature rupture of membranes, and postpartum uterine infections. This is why pregnant women with a history of premature labor or other complications may be checked for bacterial vaginosis even when they don't have any symptoms.