Metronidazole is classified as having a category B pregnancy risk. Meaning either studies done on animals showed no fetal risk OR studies done on animals showed fetal risks but then further studies done on humans showed there to be no fetal risks to humans.
So in the pharmaceutical world... metronidazole is considered to be one of the safer medicines to take.
But there is natural alternatives you can try first like plain natural yogurt (applied topically), or topical use of garlic (insert a clove into vagina for an hour or so 2 or 3 times a day until it clears up.... sew a string to the garlic clove so you don't spend all day fishing it out!), taking oral probiotics can also be helpful (eg acidophilus... the refrigerated stuff is preferred.)
Also, the link between bacterial vaginosis and preterm labour has only been demonstrated in women who have symptomatic bacterial vaginosis (eg Loads and loads of abnormal vaginal discharge and noticable odour). Because of that there's now controversy as to whether or not even testing for BV in asymptomatic pregnant women is even worthwhile as it's now being considered appropriate practice not to treat it until symptoms become present.