If you're breastfeeding and going back to work or looking for more flexibility, you may be thinking about using a breast pump. And once you start pumping, it's important to know how to safely store your expressed milk. Think about these do's and don'ts for breast milk storage.

Before you pump or handle breast milk, wash your hands with soap and water. Pumping milk also is called expressing. Store the expressed milk in a clean, capped food-grade glass container. Or use a hard plastic container that's not made with the chemical bisphenol A, also called BPA. You also can use special plastic bags designed for milk collection and storage.

Don't store breast milk in disposable bottle liners or plastic bags made for general household use.

Label each container with the date you expressed the breast milk. Use waterproof labels and ink. If you store expressed milk at your baby's child care facility, add your baby's name to the label.

Place the containers in the back of the refrigerator or freezer, where the temperature is the coolest. If you don't have access to a refrigerator or freezer, store the milk in an insulated cooler with ice packs for up to one day.

Fill separate containers with the milk your baby will need for one feeding. You might start with 2 to 4 fluid ounces (60 to 120 milliliters). Then adjust as needed. Also think about storing smaller portions — 1 to 2 ounces (30 to 60 milliliters) — for unexpected situations or delays in regular feedings. Breast milk expands as it freezes, so don't fill containers to the top if freezing.

You can add freshly expressed breast milk to refrigerated or frozen milk. But first cool the freshly expressed breast milk well. Don't add warm breast milk to frozen breast milk because it will cause the frozen milk to partly thaw.

How long you can safely keep expressed breast milk depends on how you store it. Keep in mind these general guidelines:

• Room temperature. You can keep freshly expressed breast milk at room temperature for up to six hours. But it's best to use or store the breast milk within four hours, especially if the room is warm.
• Insulated cooler. You can keep freshly expressed breast milk in an insulated cooler with ice packs for up to one day.
• Refrigerator. You can keep freshly expressed breast milk in the back of the refrigerator for up to four days. If you don't plan to use it within four days, freeze it right after pumping.
• Deep freezer. You can store freshly expressed breast milk in the back of a deep freezer for up to 12 months. But it's best to use the frozen milk within six months.

Keep in mind that research suggests the longer you store breast milk — whether in the refrigerator or in the freezer — the greater the loss of vitamin C in the milk.

It's also important to note that your breast milk changes to meet your baby's needs. Breast milk expressed when a baby is a newborn won't meet the baby's needs when the baby is a few months older.

Thaw the oldest milk first. Put the frozen container in the refrigerator the night before you intend to use it. You also can warm the milk by placing it under warm running water or in a bowl of warm water.

Don't heat a frozen bottle in the microwave or very quickly on the stove. Some parts of the milk might be too hot, and others may be cold. Also, some research suggests that rapid heating can affect the milk's antibodies.

Don't refreeze thawed breast milk. And throw away thawed milk that isn't used within 24 hours.

The color of your breast milk can change depending on what you eat. Also, thawed breast milk might seem to have a different odor or texture than freshly expressed milk. It's still safe to feed to your baby. If your baby refuses the thawed milk, shortening the storage time might help.