EVIDENCE-BASED ANSWER
Bacterial vaginosis (BV) is associated with preterm delivery (strength of recommendation [SOR]: A, meta-analysis). However, treating asymptomatic, low-risk women with BV does not always prevent preterm delivery (SOR: A, randomized controlled trials [RCTs]). There is some benefit to early screening by Gram stain using Nugent’s criteria1 (Table ) and treating BV-positive women with a history of preterm delivery, premature rupture of membranes, low birth weight infants, or spontaneous abortion. In this group, treatment has been associated with decreased rates of preterm labor, preterm prelabor rupture of membranes, and low birth weight infants (SOR: B, conflicting RCTs).
Empirically treating high-risk women without documented infection has been associated with an increase in preterm deliveries and neonatal infections (SOR: B, single RCT).
TABLE
Nugent’s Criteria
| Score | Lactobacillus morphotypes | Gardnerella and Bacteroides spp. morphotypes | Curved gram-variable rods |
|---|---|---|---|
| 0 | 4+ | 0 | 0 |
| 1 | 3+ | 1+ | 1+ or 2+ |
| 2 | 2+ | 2+ | 3+ or 4+ |
| 3 | 1+ | 3+ | |
| 4 | 0 | 4+ | |
| 1+, <1 morphotype present; 2+, 1 to 4 morphotypes present; 3+, 5 to 30 morphotypes present; 4+, >30 morphotypes present. The diagnosis of bacterial vaginosis is present with a score of 7 or greater. From Nugent 1991.1 | |||
Evidence summary
Bacterial vaginosis in early pregnancy is a risk factor for preterm delivery.2 The role of BV in preterm labor is not well understood, but it has been consistently associated with preterm labor and delivery. The detection of BV in early pregnancy seems to be a stronger risk factor for preterm delivery than BV in later pregnancy.
Studies evaluating the screening and treatment of BV in women at risk for preterm delivery have demonstrated varying results. Most treatment studies have excluded women who are in the first trimester. A meta-analysis of 7 RCTs reviewed the evidence of screening for BV in pregnancy.3 In this meta-analysis, 5 of the trials specified that women were asymptomatic, and the other 2 did not comment on whether the women were symptomatic or not. In general, there was no benefit to routine screening and treatment of BV.
However, a subgroup of high-risk women seems to benefit from screening and treatment. They defined high-risk women as those have had a preterm delivery, premature rupture of membranes, birth weight <2500 g, or spontaneous abortion. Treating BV in women with a high-risk pregnancy decreased preterm delivery (absolute risk reduction [ARR]=0.22; 90% confidence interval [CI], 0.13–0.31; number needed to treat [NNT]=4.5) regardless of antibiotic choice. However, 2 trials of high-risk women who were empirically treated for BV, but did not have BV, showed an increase in preterm delivery less than 34 weeks (number needed to harm [NNH]=11).
A new study evaluating screening for vaginal infections in pregnancy has demonstrated a reduction in preterm delivery.4 In this study, looking at a general obstetrical population in Austria, 4429 asymptomatic pregnant women between 15 and 19.6 weeks gestation had a vaginal screen for bacterial vaginosis, candidiasis and trichomoniasis. The 2048 women in the intervention group were given the results of the screen from their maternity care provider. The 2097 women in the control group and their providers did not receive the results of the vaginal screen. There were 447 women in the intervention group and 441 women in the control group with positive screens. Using the Nugent criteria, women who were diagnosed with BV received a 6-day course of intravaginal clindamycin 2% cream. Those with positive test results for Candida were treated with intravaginal clotrimazole 0.1 g; those with positive results for trichomonas received intravaginal metronidazole 500 mg for 7 days. After treatment, women with a positive test result in the intervention group had a second vaginal smear between 24 and 28 weeks. Persistent BV was treated with oral clindamycin 300 mg twice daily for 7 days. If Candida or trichomonas were noted, women were treated with the intravaginal clotrimazole or metronidazole. A statistically significant reduction was seen in preterm births in the intervention group(3.0% vs 5.3%, 95% CI, 1.2–3.6; P=.0001; number needed to screen=44).
A large study in 2000 that looked at the use of metronidazole in the treatment of asymptomatic women for BV did not demonstrate any reduction in preterm birth.5 In this study, 21,965 asymptomatic women between 8 and 22 weeks gestation were screened for BV with Gram stain using Nugent’s criteria. Then, 1953 women with BV were randomized to receive either 1 g of metronidazole orally for 2 days or placebo. Between 24 and 29 weeks, all of the women were then rescreened for BV by Gram stain. Even if the results were negative, women received another course of the metronidazole or placebo. In this study, preterm delivery rates did not improve for either low- or high-risk women. Specifically, a subgroup analysis of 213 women with previous preterm delivery did not show any benefit to treatment with metronidazole.