When using frozen breast milk or heating up formula, using an electric bottle warmer can help speed up the process and result in even, slow heat -- perfect for attaining the warm temperature that many babies love. But before you plug in your bottle warmer, make sure you read through the manufacturer's instructions to ensure you're using the warmer both safely and effectively.
Assemble and fill the baby bottle with either frozen or refrigerated breast milk or formula made with purified water. Place the assembled bottle in the bottle warmer while the warmer is both unplugged and empty.
Fill the warmer with water -- the warmer will have the appropriate fill lines indicated for the size of bottle you're using. Never fill over the appropriate fill lines or so that water is spilling over the sides of the warmer.
Plug in the warmer. You may also have to set a level for temperature. Baby bottle warmers don't heat to high temperatures as a safety precaution, but you may have two or three different levels of warmth to choose from, depending on your baby's preferences.
Leave the bottle to warm. Some will shut off immediately, while others may specify times in the manufacturer's instructions outlining how long to leave the bottle in the warmer to achieve certain temperatures. The temperature your baby prefers may require some trial and error -- as the Children's Physician Network points out, some prefer colder milk, while others like warmer milk.
Remove the bottle and test the temperature on the inside of your wrist, aiming for lukewarm formula or breast milk, suggests FamilyDoctor.org. If the bottle is too hot, leave it to cool for a few minutes until the temperature is more comfortable and try turning down the temperature level on the warmer, if possible.
Never warm a bottle in the microwave, which can result in uneven heating and hot pockets in the liquid. If you don't have a bottle warmer, you can warm a filled bottle in a deep bowl of warm water for similar, safe results.