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Bacterial Vaginosis

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Bacterial Vaginosis
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Prematurity is the leading cause of perinatal morbidity and mortality. Infection is thought to be the underlying cause in 30-40% of cases. Evidence for this comes from the presence of histologic chorioamnionitis associated with premature deliveries (Reference1) and the identification of microorganisms from the placenta, membranes (Reference2) and amniotic fluid (Reference3) of babies delivering prematurely. One such infection, which has been implicated with premature delivery in case control and cohort studies, is bacterial vaginosis (Reference1). This is not a distinct microorganism but represents an imbalance in the bacterial vaginal ecosystem resulting in a reduction in the naturally occurring lactobacilli and a consequent overgrowth of anaerobes, Gardnerella vaginalis and mycoplasma.

BV Incidence

10-30% of pregnant women

Up to 75% may be asymptomatic (Reference4)

Association with adverse pregnancy outcome

The most recent meta- analysis addressing this showed the odds ratio for preterm birth in the presence of maternal BV to be 1.85 (CI 1.62-2.11) (Reference5)

In addition the OR for preterm prelabour rupture of membranes was1.83 (CI 1.32-1.87) and for low birthweight babies 1.57 (1.39-2.44).


There are 2 main methods.

Clinical: Amsels criteria requires at least 3 out of the following to be present:

  1. Homogenous vaginal discharge
  2. Amine odour on application of discharge to 10% KOH.
  3. vaginal pH>4.5
  4. Clue cells
Gram stain

This is usually scored using either Nugent or Spiegelís criteria.

All methods have fairly good agreement. The presence of clue cells alone has good predictability but is subject to intra observer variation. Nugentís criteria has the best inter centre reliability (Reference6), although no one method is recommended for obstetric practice above all others.

Natural History

A longitudinal study found that BV is unlikely to develop after 16 weeks gestation. It spontaneously remits in 50% of positive women at term (Reference7).

One study showed a link between BV and prematurity if the BV was diagnosed at 16-20 weeks but not if diagnosed at 24-28 weeks (Reference8).


Metronidazole and clindamycin are both effective treatments. The oral preparations are recommended in obstetric practice because of a risk of increased premature deliveries in two studies using clindamycin cream (9,10) and the lack of improvement in pregnancy outcome using vaginal metronidazole preparations (11).

There has been no evidence of teratogenicity in the trials using either metronidazole or clindamycin.

Is there a benefit for screening and treating in pregnancy?

Both the Cochrane review (11) and a subsequent meta- analysis (12) found no improvement in adverse pregnancy outcome after screening and treating asymptomatic pregnant women in the general population.

High risk women with a previous preterm delivery were found to benefit from treatment in the Cochrane review. One study published subsequently found no benefit of treatment in terms of adverse pregnancy outcome in this group (13)- however they were screened and treated twice and were treated on the second time irrespective of the swab result. They therefore may have been treated when actually clear of BV. Treatment in the absence of BV has been associated with adverse pregnancy outcome (14) and this may be the confounding variable in this study.

Recommendations for practice
  1. Women with BV in pregnancy have an increased risk of preterm birth, preterm prelabour rupture of membranes and low birth weight
  2. The current evidence does not support screening and treating all women for BV in pregnancy.
  3. There appears to be clinical benefit from screening and treating those asymptomatic women who have had a previous preterm birth.
  4. Treatment should not be given if BV status is negative.
  5. Treatment should be either oral metronidazole or clindamycin
  6. Symptomatic women may also be tested and treated


1.Hillier SL, Martius J, Krohn M, Kiviat N, Holmes KK, Eschenbach DA. A case- control study of chorioamnionitis in prematurity. N Eng J Med 1988; 319: 972-8, Abstract

2.Kundsen RB, Driscoll SG, Monson RR, et al. Association of ureaplasma urealyticum in the placenta with perinatal morbidity and mortality. N Eng J Med. 1984; 310: 941, Abstract

3.Gravett MG, Hummel D, Eschenbach DA, Holmes KK. Preterm labor associated with subclinical amniotic fluid infection and with bacterial vaginosis. Obstet Gynecol 1986; 67: 229-37, Abstract

4.Mead PB. Epidemiology of bacterial vaginosis. Am J Obstet Gynecol 1993; 169: 446-449, Abstract

5.Flynn CA, Helwig AL, Meurer LN. Bacterial vaginosis in pregnancy and the risk of prematurity. A meta- analysis. The Journal of Family Practice 1999; 48: 885-892, Abstract

6.Nugent RP, Krohn MA, Hillier SL. Reliability of diagnosing bacterial vaginosis is improved by standardized method of Gram stain interpretation. J Clin Microbiol 1991; 29: 297-301, Abstract

7.Hay PE, Morgan DJ, Ison CA, Bhide SA, Romney M, McKenzie P, Pearson J, Lamont RF, Taylor-Robinson D. A longitudinal study of bacterial vaginosis during pregnancy. Br J Obstet Gynaecol. 1994; 101: 1048-1053, Abstract

8.Riduan JM, Hillier SL, Utomo B, Wilknjosastro G, Linnan M, Kandun N. Bacterial vaginosis and prematurity in Indonesia: Association in early and late pregnancy. Am J Obstet Gynecol 1992; 169: 175-178, Abstract

9. McGregor JA, French JI, Jones W et al. Bacterial vaginosis is associated with prematurity and vaginal fluid mucinase and sialidase: results of a controlled trial of topical clindamycin cream. Am J Obstet Gynecol 1994; 170: 1048-60, Abstract

10. Joesoef MR, Hillier SL, Wiknjosastro G, et al. Intravaginal clindamycin treatment for bacterial vaginosis: effects on preterm delivery and low birth weight. Am J Obstet Gynecol 1995; 173; 1527-31, Abstract

11. Brocklehurst P, Hannah M, McDonald H. Interventions for treating bacterial vaginosis in pregnancy (Cochrane Review). In: The Cochrane Library, Issue1, 2002.Oxford: Update Software, Abstract

12. Guise J-M, Mahon SM, Aickin M, Helfand M, Peipert JF, Westhoff C. Screening for bacterial vaginosis in pregnancy. Am J Prev Med 2001; 20: 62-72, Abstract

13. Carey JC, Klebanoff MA, Hauth JC et al. Metronidazole to prevent preterm delivery in pregnant women with asymptomatic bacterial vaginosis. National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine units. N Eng J Med 2000; 342: 535-40, Abstract

14. Hauth JC, Goldenberg RL, Andrews WW, DuBard MB, Cooper RL. Reduced incidence of preterm delivery with metronidazole and erythromycin in women with bacterial vaginosis. N Eng J Med 1995; 333: 1732-6, Abstract

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