It is important to heat the bottles of formula safely. Formula milk (like breast milk) only needs to be at body temperature (or lukewarm). The most common way that parents will heat bottles is by placing them in a jug, or saucepan, of warm to hot water. The water does not need to be boiling, hot water out of the tap is fine. Heating usually only takes a few minutes, and no more than 10 minutes. The heated milk needs to be used within 1 hour of heating (as bacteria can multiply in warmed milk).
Give the bottle a gentle shake after heating, to mix the heated milk through and ALWAYS test the temperature of the milk on the inside of your arm before giving it to your baby. It should feel comfortably warm (or lukewarm).
Image 11-84 shows how to test the tempertaure of the formula milk.
Microwaving bottles is not generally recommended. The microwaving can change the nutritional composition of the milk (particularly proteins) and make it an inadequate source of nutrition for your baby (especially if used all the time). The milk also heats unevenly and can burn your baby's mouth. If you do decide to heat up the odd bottle in the microwave, always heat the milk without the cap and lid on the bottle (so it doesn't 'explode'), and always give it a good shake and test it on your arm, before giving it to your baby, to make sure it is not too hot.
Some parents buy special bottle warmers. If you have one, follow the instructions of the manufacturer and be careful not to leave the bottles in the warmer for prolonged periods (more than 30 minutes to an hour).