Metronidazole is not used in early pregnancy because of potential adverse effects on the fetus. You are already 30 weeks preganacy, so you don't need to worry about it. Details info pls visit FDA and consult your Ob specialist or you can search the drug info from the internet (google).
I tried several types of antobiotics, including oral and vignal medications during my pregnancy. At that moment, I had a same question about the safety of those drugs. Although my OB and pharmcist said those drugs were safe, I was still worried about that.
My baby was born 2 months ago, I don't see what's wrong with her right now. Therefore, you don't need to worry.