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Thanks for writing to us with your health concern.
Let me elaborate a few points which will reassure you, backing up my statements with studies attached, which maybe in medical terminology, but the gist of which will be grasped by a perceptive patient like you.
1. BV ( bacterial vaginosis ) is known for recurrence. Almost 30 % women relapse in the first month of therapy. Apart from sexual partners, other factors are being incriminated - stress, lack of sleep, vaginal douching or over cleaning with various agents etc.
2. During pregnancy too, recurrence is common, and recurrent BV has an association with adverse pregnancy outcomes, but by far, the number of successful pregnancies outnumbers the adverse outcomes. IN your case, with a prior history of preterm labour and your age ( not specified by you in the query ), there is more room for caution.
3. Metronidazole ( flagyl ) is completely safe in pregnancy, yes, even a third course. Rest assured.
4. It is better to combine topical therapy ( clindamycin cream ) with oral Flagyl, please discuss this with your OB.
Am attaching a few links, please go through them in detail -
http://epirev.oxfordjournals.org/content/24/2/102.full
http://jac.oxfordjournals.org/content/44/6/854.full
http://www.ncbi.nlm.nih.gov/pubmed/7856680
Take care, and please feel free to discuss further.
Regards