Preterm birth is one of the most common causes of neonatal morbidityand mortality. The etiology of spontaneous preterm birth is multifactorial;however, there is a strong evidence support that subclinical infection of the fetal membranes and abnormal colonization of the genital tract areassociated with subsequent preterm labor in up to 40% of cases.Ascending infection from the vagina is the most likely source ofinfection that causes preterm birth; Research has focused on bacterialvaginosis (BV), the syndrome in which the normal vaginal lactobacilli arereplaced by a mixed flora with high concentrations of anaerobic bacteria.Current evidence support screening and treatment of such infections inhigh–risk women complaining of preterm labor pains, as it treats currentpreterm birth and prevent a portion of these women from having anotherpreterm birth.