The full, clinically endorsed recommendation should be obtained from Lareb.   

Rationale for drug selection  

Metronidazole is used in pregnancy to treat infections, including bacterial vaginosis and trichomoniasis, and as perioperative prophylaxis. It is known that pregnancy alters pharmacokinetics. Therefore, dose adjustments might be needed to maintain efficacy.

Pharmacokinetics of metronidazole in pregnancy 

Metronidazole undergoes hepatic metabolism via CYP3A4 and CYP2C9. The activity of these enzymes is elevated during pregnancy which can lead to lower metronidazole concentrations. The drug crosses the placenta, and cord concentrations are slightly lower than maternal plasma levels. However, the evidence of lower metronidazole concentrations during pregnancy is limited. Maternal-foetal PBPK model simulations of metronidazole were not of sufficient quality to inform alternative dosing strategies or support clinical decision-making.

In short 

Pregnancy-induced pharmacokinetic changes may alter metronidazole exposure. However, there is insufficient evidence to confirm decreased metronidazole concentrations during pregnancy. Consult Lareb for dosing recommendations.