Breast milk storage can be confusing. Follow these practical tips on choosing containers, freezing breast milk, thawing breast milk and more.

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

What kind of container should I use to store expressed breast milk?

Before expressing or handling breast milk, wash your hands with soap and water. Then store the expressed milk in a clean, capped food-grade glass container or hard plastic container that's not made with the chemical bisphenol A (BPA). You can also use special plastic bags designed for milk collection and storage.

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

What's the best way to store expressed breast milk?

Using waterproof labels and ink, label each container with the date you expressed the breast milk. If you're storing expressed milk at your baby's childcare 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 temporarily in an insulated cooler with ice packs.

Fill individual containers with the milk your baby will need for one feeding. You might start with 2 to 4 ounces (60 to 120 milliliters), and then adjust as needed. Also consider 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 brim.

Can I add freshly expressed breast milk to already stored milk?

You can add freshly expressed breast milk to refrigerated or frozen milk. However, thoroughly cool the freshly expressed breast milk in the refrigerator or a cooler with ice packs before adding it to previously chilled or frozen milk. Don't add warm breast milk to frozen breast milk because it will cause the frozen milk to partially thaw.

How long does expressed breast milk keep?

How long you can safely keep expressed breast milk depends on the storage method. Consider these general guidelines:

• Room temperature. Freshly expressed breast milk can be kept at room temperature for up to six hours. However, it's optimal to use or properly store the breast milk within four hours, especially if the room is warm.
• Insulated cooler. Freshly expressed breast milk can be stored in an insulated cooler with ice packs for up to one day.
• Freshly expressed breast milk can be stored in the back of the refrigerator for up to four days in clean conditions. However, it's optimal to use or freeze the milk within three days.
• Deep freezer. Freshly expressed breast milk can be stored in the back of a deep freezer for up to 12 months. However, using the frozen milk within six months is optimal.

Keep in mind that research suggests that 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 as completely meet the same baby's needs when he or she is a few months older. Also, storage guidelines might differ for preterm, sick or hospitalized infants.

How do I thaw frozen breast milk?

Thaw the oldest milk first. Place the frozen container in the refrigerator the night before you intend to use it. You can also gently 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 cold. Also, some research suggests that rapid heating can affect the milk's antibodies.

More research is needed on whether previously frozen milk that's been thawed can be frozen again and safely used. However, many experts recommend discarding thawed milk that isn't used within 24 hours.

Does thawed breast milk smell or look different from fresh breast milk?

The color of your breast milk might vary slightly depending on your diet. Also, thawed breast milk might seem to have a different odor or consistency 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.