Metronidazole has been found to be tumorgenic in rats/hamsters...etc. Mostly affecting the lung and liver.
This type of complication has not been demonstrated in humans.
This medicine is secreted in breast milk with the level in the milk about the same as that in the mother's blood.
This medicine is recommended to breast feeding women when the benefits outweight the risks, which is a conversation nursing mothers must have with their doctors.
Lastly, this medicine is category B in pregnancy and used frequently on pregnant women.
Since I am not your doctor I cannot give you a recommendation either way. The information above are just the facts. You must talk to your doctor about taking this medicine, especially if you are uncomfortable about it.