If you’re bottle feeding your baby and want the bottle to be warm, Dr. Benjamin Spock, author of Dr. Spock’s Baby and Child Care: The One Essential Parenting Book advises that baby bottles should never be heated in a microwave oven. The reason he has cited is that the contents may be hot enough to burn the baby even when the bottle feels cool to your touch.
If it’s really unavoidable (yeah, there are a lot of reasons when you really don’t want to), make sure that you stir the formula well with a spoon so that there will be no hot spots in the formula. Drop a few drops on the inside your wrist. If the formula feels hot, then it is too hot for the baby.
The best way to warm the bottle is warming it in a saucepan, a pitcher of hot water, or a washbasin. You can also use an electric bottle warmer if you don’t have a hot-water faucet at home.