This drug should be used during pregnancy only if clearly needed.
-According to some authorities: Use is not recommended.
-For Acea (0.75% topical gel): Safety has not been established; use is not recommended unless considered essential by physician.
-For Zidoval (0.75% vaginal gel): According to some authorities, caution is recommended.

AU TGA pregnancy category: B2
US FDA pregnancy category:
-For most products: B
-For 1% topical cream and 1.3% vaginal gel: Not assigned.

Risk summary: No data available on use of this drug in pregnant women to inform a drug-related risk.

Animal studies with topical formulations have not been reported. Animal studies with the oral formulation have failed to reveal evidence of impaired fertility, teratogenicity, or fetal harm at higher doses; however, some intrauterine deaths were observed in mice when the drug was administered intraperitoneally. There has also been evidence of oral metronidazole acting as a carcinogen in rodents. Oral metronidazole readily crosses the placenta and rapidly enters fetal circulation. Published data from studies include more than 5000 pregnant women who used systemic metronidazole during pregnancy (many studies included first trimester exposures); in 1 study, increased risk of cleft lip (with or without cleft palate) was shown in infants exposed to metronidazole in utero, but these findings were not confirmed. Most studies did not show an increased risk for congenital anomalies or other adverse fetal outcomes after metronidazole exposure during pregnancy. There are no controlled data in human pregnancy with this topical drug.

AU TGA pregnancy category B2: Drugs which have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals are inadequate or may be lacking, but available data show no evidence of an increased occurrence of fetal damage.

US FDA pregnancy category B: Animal reproduction studies have failed to demonstrate a risk to the fetus and there are no adequate and well-controlled studies in pregnant women.

US FDA pregnancy category Not Assigned: The US FDA has amended the pregnancy labeling rule for prescription drug products to require labeling that includes a summary of risk, a discussion of the data supporting that summary, and relevant information to help health care providers make prescribing decisions and counsel women about the use of drugs during pregnancy. Pregnancy categories A, B, C, D, and X are being phased out.

See references

Neither topical nor vaginal formulations of metronidazole have been studied during breastfeeding. After vaginal administration, plasma levels are less than 2% of those after a 500 mg oral dose; after topical administration, blood levels are about 1% of the Cmax after a 250 mg oral dose. According to some experts, topical or vaginal use of metronidazole during breastfeeding is unlikely to be of concern.

In a study of nursing mothers taking oral metronidazole 600 (n=11) or 1200 (n=4) daily, maternal plasma levels averaged 5 and 12.5 mcg/mL, respectively, within 2 hours after dosing; the milk-to-maternal plasma ratio was about 1.

The ratio of serum levels of Zidoval vaginal gel/oral metronidazole is about 0.02; metronidazole is excreted in milk at levels similar to those in maternal serum and the ratio of serum levels of metronidazole in the breastfed infant/mother is about 0.15.

A decision should be made to discontinue breastfeeding or discontinue the drug, taking into account the importance of the drug to the mother.
-For 1.3% vaginal gel: Breastfeeding is not recommended during use of this drug and for 2 days after therapy ends.
-For Acea (0.75% topical gel): Safety has not been established; use is not recommended unless considered essential by physician.
-For Zidoval (0.75% vaginal gel): According to some authorities, caution is recommended.

Excreted into human milk: Unknown
Excreted into animal milk: Data not available

Comments:
-After oral administration, metronidazole is secreted in breast milk in levels similar to those found in the plasma; metronidazole blood levels from topical/vaginal administration are significantly lower than blood levels attained after oral administration.
-Since some drug is systemically absorbed after vaginal administration, excretion in human milk after topical use is possible.
-A nursing mother may choose to pump and discard her milk during therapy and for 24 to 48 hours after therapy ends (depending on product) and feed her infant stored human milk or formula.

See references