Intrauterine infection has a strong association with spontaneous preterm deliveries.1 Bacterial vaginosis (BV) is a frequent and modifiable risk factor.1 BV has been associated with numerous obstetric and gynecologic complications, including preterm labor and delivery, preterm premature rupture of membranes, spontaneous abortion, chorioamnionitis, postpartum endometritis, post-cesarean delivery wound infections, postsurgical infections, and subclinical pelvic inflammatory disease.2 BV is characterized by altered vaginal flora. There is diminished or absent flora of lactobacilli, resulting in increased vaginal pH, and a significantly increased colonization of several anaerobic or facultative microorganisms, especially Gardenerella vaginalis, Prevotella sp, Bacteroides sp, Mobiluncus sp, gram positive cocci, and genital mycoplasma (Mycoplasma hominis and Ureaplasma urealyticum).1

The characteristic pH of women of reproductive age is 3.5 to 4.5.3 It has been shown that women with a vaginal pH > 5.0 or > 4.5 and a Nugent score of 9–10 have significantly increased rates of preterm birth and birthweigth <2500 g or <1500 g.1 It is, therefore, important to maintain the ideal vaginal pH in order to prevent these complications. Additionally, maintenance of a 3.5 to 4.5 acidic pH serves as a vaginal protection system that renders the conditions unfavorable for the proliferation of potentially pathogenic organisms, while providing a suitable environment to acidophilus bacilli.3

These bacilli metabolize under this situation and produce large amounts of lactic acid by glucose anaerobic fermentation. Lactic acid keeps its acidic pH and prevents uncontrolled growth of other bacteria present in the vagina.3 Therefore, the characteristic pH of women of reproductive age, 3.5 - 4.5, is achieved due to the presence of a sufficient amount of lactic acid in the vagina.3

Lactic acid is thus the key element for the maintenance of vaginal health. It inhibits the growth of fungi, protozoa, Haemophilus and other pathogenic bacteria, which generally need a pH > 6.0. Lactic acid compound produces hydrogen peroxide and favors the action of bacteriocins, which eventually hinders the proliferation of a wide range of pathogens. Acidifying the vagina with lactic acid is a potential therapeutic intervention to prevent and treat infections and maintain ideal pH.3

References

1. Krauss-Silva L, Almada-Horta A, Alves MB, et al. Basic vaginal pH, bacterial vaginosis and aerobic vaginitis: prevalence in early pregnancy and risk of spontaneous preterm delivery, a prospective study in a low socioeconomic and multiethnic South American population. BMC Pregnancy and Childbirth 2014;14:107.
2. Yudin MH, Money DM, Boucher M, et al. Screening and Management of Bacterial Vaginosis in Pregnancy. Available from: https://sogc.org/wp-content/uploads/2013/01/gui211CPG0808.pdf.
3. Haya J, García A, López-Manzanara C, et al. Importance of Lactic Acid in Maintaining Vaginal Health: A Review of Vaginitis and Vaginosis Etiopathogenic Bases and a Proposal for a New Treatment. Open Journal of Obstetrics and Gynecology 2014;4:787-799.