If your baby has finished a bottle and has a little formula or breastmilk left over, its best to throw it out, and every time your baby has finished a bottle, you need to clean it thoroughly. If you are using formula, make sure to follow the directions on the package when preparing it, and boil the water for one to two minutes before using it even if the water is bottled rather than tap to protect your baby from anything that may be lurking in the water supply. When you are pumping milk for your baby, date the bottle or plastic liner you express into, refrigerate it, and make sure to use the milk within three or four days. Kept at room temperature, milk is usable for only 12 to 24 hours, but if you freeze the milk, at zero degrees, it can keep for as long as three to six months, though it does lose some of its antibodies. When you are thawing frozen milk, or heating refrigerated milk, make sure to rely on your stovetop rather than your microwave.
Microwaves heat unevenly.
On your stovetop, heat water to below boiling point with the bottle immersed, or use a bottle warmer if you are so inclined. There is no reason you need to warm milk or formula before a feeding, but some babies prefer it warm because it reminds them of breastmilk direct from mom. Shake the bottle before testing the temperature for an accurate test and a redistribution of the cream layer, and your baby will have the perfect meal. Until your baby is on a schedule, you should offer to feed them every two to three hours, or whenever they seem hungry. To prevent their taking in too much air, hold the bottle at a 45 degree angle and make sure that the nipple and neck of the bottle are always full.