Metronidazole in Pregnancy
Direct Answer
Metronidazole can be used safely during pregnancy, but the timing and indication matter: avoid it in the first trimester when possible, use it confidently in the second and third trimesters for bacterial vaginosis and trichomoniasis, and choose clindamycin vaginal cream as the preferred first-trimester alternative. 1, 2
First Trimester: Avoid Metronidazole
Clindamycin vaginal cream 2% (one full applicator intravaginally at bedtime for 7 days) is the CDC-recommended first-line treatment for bacterial vaginosis in the first trimester. 2
The American College of Obstetricians and Gynecologists notes that oral metronidazole is contraindicated during the first trimester due to concerns about potential teratogenicity, although meta-analyses do not indicate teratogenicity in humans. 2
If clindamycin vaginal cream is contraindicated, oral clindamycin 300 mg twice daily for 7 days can be used as an alternative to minimize systemic exposure during the first trimester. 2
Important First-Trimester Caveat
Do not use metronidazole gel intravaginally during the first trimester—existing data do not support the use of topical metronidazole agents during pregnancy at this stage. 2
Clindamycin cream is oil-based and may weaken latex condoms and diaphragms; counsel patients to use non-latex barrier methods during treatment and for several days afterward. 2
Second and Third Trimesters: Metronidazole Is Safe and Preferred
The CDC recommends oral metronidazole 250 mg three times daily for 7 days as the preferred treatment for bacterial vaginosis during the second and third trimesters. 1, 2
Alternative regimens include:
Meta-analyses of human studies show no statistically significant increase in preterm birth, low birth weight, or congenital anomalies after metronidazole exposure in the second or third trimester. 1
The U.S. Food and Drug Administration classifies metronidazole as Pregnancy Category B, meaning animal studies have not demonstrated fetal risk and there are no adequate human studies to suggest harm. 1
Trichomoniasis Treatment After First Trimester
Standard therapy after the first trimester is a single oral dose of metronidazole 2 g. 1
Alternative regimen: metronidazole 500 mg orally twice daily for 7 days. 1
Trichomoniasis is associated with premature rupture of membranes and preterm delivery, making treatment important. 1
Critical Clinical Pitfalls
Avoid Clindamycin Vaginal Cream in Later Pregnancy
Do not use clindamycin vaginal cream in the second and third trimesters. Three clinical trials reported higher rates of prematurity and neonatal infections when the cream was used later in pregnancy. 1, 2
This contraindication applies specifically to clindamycin vaginal cream, not oral clindamycin, which remains an acceptable alternative throughout pregnancy. 1
Alcohol Counseling
- Advise patients to abstain from alcohol during metronidazole therapy and for at least 24 hours after the last dose to prevent disulfiram-like reactions (flushing, nausea, vomiting, tachycardia). 1
Partner Treatment Not Recommended for BV
- Treating male sexual partners routinely is not recommended for bacterial vaginosis, as partner therapy does not improve maternal treatment response or reduce recurrence rates. 1, 2
Why Treatment Matters: Untreated Infections Increase Morbidity
Untreated bacterial vaginosis is linked to premature rupture of membranes, preterm labor, preterm birth, postpartum endometritis, and post-cesarean wound infection, all of which increase maternal-neonatal morbidity. 1
Treating BV in high-risk pregnant women (those with a prior preterm delivery) may lower the incidence of preterm delivery. 1
All symptomatic pregnant women should be tested and treated for bacterial vaginosis to prevent these serious adverse pregnancy outcomes. 2
Follow-Up Management
Routine follow-up visits are not required if BV or trichomoniasis symptoms have resolved after treatment. 1
For pregnant women classified as high-risk (history of preterm delivery), a follow-up assessment approximately one month after completing therapy can be considered to confirm eradication. 1, 2
Addressing Conflicting Evidence
One 2023 animal study in rats suggested maternal and fetal hepatotoxicity and teratogenicity with metronidazole. 3 However, this finding is not supported by nearly four decades of human data and multiple meta-analyses showing no teratogenic effects in humans. 4, 5
A 2021 review noted a 70% increased risk of spontaneous abortion associated with metronidazole use, but this risk should be interpreted cautiously as it may be confounded by the severity of the underlying genitourinary infection rather than the medication itself. 5
The weight of guideline evidence from the CDC and ACOG, combined with extensive human safety data, supports the safe use of metronidazole in the second and third trimesters. 6, 1, 2