Infection within the womb (intrauterine infection) is strongly associated with spontaneous PTB. The commonest pathway for intrauterine infection is the ascent of unhealthy bacteria from the vagina and cervix into the womb. Bacterial vaginosis (BV), in which the normally dominant healthy bacteria in the vagina (lactobacilli) are replaced with unhealthy bacteria, is strongly associated with PTB. Lactobacilli, principally the strains that produce higher levels of the chemical hydrogen peroxide, appear to protect against BV and reduce the risk of PTB.