It's not safe to reheat a bottle of breast milk. Give your baby an hour to finish up, and then dump whatever’s left. Once a baby sucks on the bottle, it’s contaminated with saliva and is a breeding ground for bacteria.
Note: Babies don't require warm milk (whether it's formula or breast milk).
Have more bottle-feeding questions? Download our chart here and put it on the fridge—it has the answer to all your questions.