Pregnancy Care Guidelines

Asymptomatic bacterial vaginosis

Bacterial vaginosis results from the relative deficiency of normal Lactobacillus species in the vagina and relative overgrowth of anaerobic bacteria. Identifying and treating bacterial vaginosis does not appear to change the risk of preterm birth or other pregnancy complications.

As identifying and treating asymptomatic bacterial vaginosis does not appear to change the risk of preterm birth or other pregnancy complications, routine testing in pregnancy is not appropriate.

45.1 Background

Bacterial vaginosis results from the relative deficiency of normal Lactobacillus species in the vagina and relative overgrowth of anaerobic bacteria. This reduces the normal acidity of the vagina. Bacterial vaginosis is asymptomatic for 50% of women in pregnancy (Joesoef & Schmid 2002) but may result in a vaginal discharge that can be grey in colour with a characteristic ’fishy’ odour (McDonald et al 2007).

45.1.1 Asymptomatic bacterial vaginosis in pregnancy

  • Several large prospective, longitudinal studies have found the prevalence of bacterial vaginosis to be in the range 9–23% Hillier et al 1992, Hillier et al 1995, Meis et al 1995, Goldenberg et al 1996, Goldenberg et al 1998, Pastore et al 1999. A study in remote central Australia (n=205) Smith et al 2005 found a prevalence of 26–36% among women attending clinics for a women’s health assessment (for either a symptomatic episode or routine check but not for antenatal care).
  • Risk factors: Bacterial vaginosis in pregnancy is more common among women of low socioeconomic status and women who had low birth weight babies in previous pregnancies (Hillier et al 1995, French et al 2006).

45.1.2 Risks associated with asymptomatic bacterial vaginosis in pregnancy

Bacterial vaginosis has been associated with an increased risk of preterm birth McGregor et al 1990, Kurki et al 1992, Hay et al 1994, Hillier et al 1995, with a review of case–control and cohort studies finding that women with bacterial vaginosis were 1.85 times more likely (95% CI 1.62–2.11) to give birth preterm than women without (Flynn et al 1999). The higher risk of preterm birth remains in women diagnosed with bacterial vaginosis early in pregnancy, even if the bacterial vaginosis spontaneously resolves later in pregnancy (Gratacos et al 1998).

45.2 Testing for asymptomatic bacterial vaginosis

Routine testing for asymptomatic bacterial vaginosis in pregnancy is not recommended in the United Kingdom NICE 2008, the United States USPSTF 2008 or Canada SOGC 2008. The systematic review supporting the United States statement (Nygren et al 2008) found that:

  • no studies directly addressed the adverse effects of testing pregnant women who are asymptomatic for bacterial vaginosis
  • there is no clear benefit for the general population from testing and treating asymptomatic bacterial vaginosis during pregnancy
  • although a subgroup of high-risk women may benefit from testing and treatment for bacterial vaginosis in pregnancy, a sizeable group would receive either no benefit or may experience harm.

45.2.1 Diagnosis of bacterial vaginosis

Bacterial vaginosis is generally diagnosed by either:

  • Amsel’s criteria (thin white-grey homogenous discharge, pH greater than 4.5, release of ‘fishy odour’ on adding alkali, clue cells present on direct microscopy) (Amsel et al 1983); or
  • Nugent’s criteria (Gram-stained vaginal smear to identify proportions of bacterial morphotypes with a score of greater than six indicating bacterial vaginosis) (Nugent et al 1991), which has both high sensitivity and specificity Nelson et al 2003, Taylor-Robinson et al 2003, Hogan et al 2007, Nelson et al 2007, does not seem to vary with the vaginal site of collection (Culhane et al 2005) and has greater sensitivity than standard antenatal clinical diagnosis or a commercial test (Hogan et al 2007).

Other forms of testing, including the pH/whiff test or QuickVue Advanced pH and Amines test, have not been found to be reliable in detecting asymptomatic bacterial vaginosis in pregnancy Charonis & Larsson 2006, Nelson et al 2007.

45.2.2 Effect of treatments on risks associated with bacterial vaginosis

While antibiotics are effective in eradicating bacterial vaginosis (McDonald et al 2007), treatment does not change the risk of preterm birth, low birth weight or premature rupture of the membranes in women at low risk of preterm birth. The Cochrane review (McDonald et al 2007) found:

  • no statistically significant decrease in the risk of preterm birth at less than 37 weeks gestation for any treatment versus no treatment or placebo (n=5,888; OR 0.91; 95% CI 0.78–1.06)
  • no evidence of an effect on birth before 34 weeks (n=851; OR 1.22; 95% CI 0.67–2.19) or birth before 32 weeks (n=3,565; OR 1.14; 95% CI 0.76–1.70)
  • no difference in the incidence of low birth weight (n=4,107; OR 0.95; 95% CI 0.77–1.17)
  • no decrease in the risk of preterm rupture of membranes (n=2,579; OR 0.88; 95% CI 0.61–1.28)
  • a possible reduction in risk of preterm birth if treatment is given before 20 weeks pregnancy (n=2,387; OR 0.72; 95% CI 0.55–0.95).

In women with a previous preterm birth, treatment did not affect the risk of subsequent preterm birth (n=622; OR 0.83; 95% CI 0.59–1.17). However, two small studies showed a decrease in the risk of preterm rupture of the membranes (OR 0.14; 95% CI 0.05–0.38) and low birth weight (OR 0.31; 95% CI 0.13–0.75; n=114).

Although there is no evidence that testing and treating all women with bacterial vaginosis in the antenatal period will have a major impact on the rate of preterm birth, there is emerging evidence that early treatment may be more effective (McDonald et al 2007).


  • Grade B
  • 50

Do not routinely offer pregnant women testing for bacterial vaginosis.

Approved by NHMRC in December 2011; expires December 2016


  • Practice point
  • AAA

Early treatment (before 20 weeks pregnancy) of proven bacterial vaginosis may be beneficial for women with a previous preterm birth.

Approved by NHMRC in December 2011; expires December 2016

45.3 Practice summary: testing for bacterial vaginosis


In the antenatal period.


  • Midwife
  • GP
  • obstetrician
  • Aboriginal and Torres Strait Islander health worker
  • multicultural health worker.


  • Document and follow-up
    If a woman is tested for bacterial vaginosis, note the results in her record. Have a system in place so that women who test positive are given information about treatments.

45.4 Resources


  • Amsel R, Totten PA, Spiegel CA (1983) Nonspecific vaginitis: diagnostic criteria and microbial and epidemiological associations. Am J Med 74: 14–22.
  • Charonis G & Larsson PG (2006) Use of pH/whiff test or QuickVue Advanced pH and Amines test for the diagnosis of bacterial vaginosis and prevention of postabortion pelvic inflammatory disease. Acta Obstet Gynecol Scand 85(7): 837–43.
  • Culhane JF, Desanto D, Goldenberg RL et al (2005) Variation in Nugent score and leukocyte count in fluid collected from different vaginal sites. Obstet Gynecol 105(1): 120–23.
  • Flynn CA, Helwig AL, Meurer LN (1999) Bacterial vaginosis in pregnancy and the risk of prematurity: a meta-analysis. J Fam Pract 48: 885–92.French JI, McGregor JA, Parker R (2006) Readily treatable reproductive tract infections and preterm birth among black women. Am J Obstet Gynecol 194(6): 1717–26.
  • Goldenberg RL, Iams JD, Mercer BM et al (1998) The preterm prediction study: the value of new vs. standard risk factors in predicting early and all spontaneous preterm births. NICHD MFMU Network. Am J Public Health 88(2): 233–38.
  • Goldenberg RL, Klebanoff M, Nugent RP et al (1996) Bacterial colonization of the vagina during pregnancy in four ethnic groups. Am J Obstet Gynecol 174: 1618–21
  • Gratacos E, Figueras F, Barranco M et al (1998) Spontaneous recovery of bacterial vaginosis during pregnancy is not associated with an improved perinatal outcome. Acta Obstet Gynecol Scand 77: 37–40.
  • Hay PE, Morgan DJ, Ison CA et al (1994) A longitudinal study of bacterial vaginosis during pregnancy. Brit J Obstet Gynaecol 101: 1048–53.
  • Hillier S, Krohn MA, Nugent RP et al (1992) Characteristics of the three vaginal flora patterns assessed by gram stain among pregnant women. Am J Obstet Gynecol 166: 938–44.
  • Hillier S, Nugent RP, Eschenbach D et al (1995) Association between bacterial vaginosis and preterm delivery of a low-birth-weight infant. N Engl J Med 333: 1737–42.
  • Hogan VK, Culhane JF, Hitti J et al (2007) Relative performance of three methods for diagnosing bacterial vaginosis during pregnancy. Maternal Child Health J 11(6): 532–39.
  • Joesoef M & Schmid G (2002) Bacterial vaginosis. Clin Evidence 7: 1400–08.
  • Kurki T, Sivonen A, Renkonen O-V et al (1992) Bacterial vaginosis in early pregnancy and pregnancy outcome. Obstet Gynecol 80: 173–77.
  • McDonald HM, Brocklehurst P, Gordon A (2007) Antibiotics for treating bacterial vaginosis in pregnancy. Cochrane Database of Systematic Reviews 2007, Issue 1. Art. No.: CD000262. DOI: 10.1002/14651858.CD000262.pub3.
  • McGregor JA, French JI, Richter R et al (1990) Antenatal microbiological maternal risk factors associated with prematurity. Am J Obstet Gynecol 163: 1465–73.
  • Meis P, Goldene, Mercer B et al (1995) The preterm prediction study: significance of vaginal infections. Am J Obstet Gynecol 173: 1231–35.
  • Nelson DB, Bellamy S, Gray TS et al (2003) Self-collected versus provider-collected vaginal swabs for the diagnosis of bacterial vaginosis: An assessment of validity and reliability. J Clin Epidemiol 56: 862–66.
  • Nelson DB, Bellamy S, Nachamkin I et al (2007) First trimester bacterial vaginosis, individual microorganism levels and risk of second trimester pregnancy loss among urban women. Fertil Steril 88(5): 1396–403.
  • NICE (2008) Antenatal Care. Routine Care for the Healthy Pregnant Woman. National Collaborating Centre for Women’s and Children’s Health. Commissioned by the National Institute for Health and Clinical Excellence. London: RCOG Press.
  • Nugent RP, Krohn MA, Hillier SL (1991) Reliability of diagnosing bacterial vaginosis is improved by a standardised method of Gram stain interpretation. J Clin Microbiol 29: 297–301.
  • Nygren P, Fu R, Freeman M et al (2008) Evidence on the benefits and harms of screening and treating pregnant women who are asymptomatic for bacterial vaginosis: an update review for the U.S. Preventive Services Task Force. Ann Intern Med 148(3): 220–33.
  • Pastore LM, Royce RA, Jackson TP et al (1999) Associations between bacterial vaginosis and fetal fibronectin at 24–29 weeks’ gestation. Obstet Gynecol 93: 117–23.Smith KS, Tabrizi SN, Fethers KA et al (2005) Comparison of conventional testing to polymerase chain reaction in detection of Trichomonas vaginalis in indigenous women living in remote areas. Int J STD AIDS 16: 811–15.
  • SOGC (2008) Screening and Management of Bacterial Vaginosis in Pregnancy. SOGC Clinical Guideline No 211. Society of Obstetricians and Gynaecologists of Canada.
  • Taylor-Robinson D, Morgan DJ, Sheehan M et al (2003) Relation between Gram-stain and clinical criteria for diagnosing bacterial vaginosis with special reference to Gram grade II evaluation. Int J STD AIDS 14(1): 6–10.
  • USPSTF (2008) Screening for bacterial vaginosis in pregnancy to prevent preterm delivery: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med 148(3): 214–19.
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