I am going out of town and need to leave my baby with a sitter. Can I leave breast milk for her? How long will it stay good and how should I store it?
Three factors come to mind when preparing to travel without a breastfeeding infant. The first is how to stockpile extra milk for when you are away. The second is how to store it. And the third is how to keep up your milk supply while you are traveling.
Stockpiling is best done by pumping milk ahead of time. For many women, before the baby wakes up or about an hour after the first morning feed is their most productive time for pumping. Some moms can get lots of milk, while for others pumping just isn't the same as nursing the baby, and not much is obtained. It will help to get a good running start with pumping so you become used to it and get a feel for how much milk you can expect. Consumer Reports has a nice article on the multitude of breast pump options.
Breast milk can be stored in the refrigerator for up to five days. If you are going to store it longer than that, it should be frozen. Most moms store the milk in clean glass or plastic jars in one-feeding volumes. Sometimes a few 1-ounce bottles are good to store as well, so if the baby is still hungry a whole extra feeding doesn't need to be defrosted. Defrosted milk can be kept 24 hours in the refrigerator and up to four hours at room temperature. Milk should be defrosted by putting it in the refrigerator or sitting the container in warm water. Never use the microwave because nutrients are lost, and hot pockets of milk can burn the baby. For more information see the Centers for Disease Control (CDC) website.
Lastly, keeping up your milk supply while traveling can be a challenge. Some moms bring their pump with them, and either save the milk, FedEx it home on dry ice, or practice "pump and dump" where the milk is discarded. Some moms find that hand expressing milk works fine for them when travelling, especially if they aren't trying to keep the milk. Manual breast pumps are sometimes easier than hauling around an electric pump, but some moms do best with their regular electric pump. Before you travel, play around with the options and think about the constraints of your environment, so you know what is likely to work best for you.
For more information, I have a whole section on travel after childbirth in The Working Woman's Pregnancy.