By Alison Heyerdahl

Breastfeeding can be daunting for even the most seasoned mother, and pumping or expressing milk can be even more difficult.  Knowing that breastmilk is best for their babies, working mothers (and those whose babies can’t latch) often try to maximize their breastmilk stockpiles to avoid going the formula route.  

To save fridge and freezer space, combining milk from different pumping sessions makes sense, but is it a safe option?  The answer is generally ‘yes,’ but certain guidelines need to be followed.

How to combine breastmilk from different pumping sessions

To reduce the chances of bacterial growth, freshly pumped breastmilk should first be chilled to the same temperature as the milk stored in the bottle. Adding warm breastmilk to older, previously chilled breastmilk can increase the chances of bacterial growth.  

As long as the freshly pumped breastmilk is sufficiently chilled, it can be added to a bottle of milk pumped earlier that day and even to refrigerated milk from a pumping session the previous day.  Two bottles or containers of refrigerated milk can also be safely combined for storage in the freezer.  Ensure that the containers and pump are clean before expressing milk to prevent breast milk contamination. 

How long can combined breastmilk be stored?

The longer breastmilk is stored, the more nutrition the milk loses.  It also starts to lose its ability to fight off bacterial growth.  To avoid confusion about which milk was stored, always label the containers with the child’s name and date.  When adding fresh milk to previously-stored milk, use the date already on the container as a guide for when the milk should be used.  The following rules apply for safely storing breastmilk:

• Fresh breastmilk can be stored at room temperature for 4 hours (depending on how warm it is)

• Fresh breastmilk can be stored in the refrigerator for 3 – 6 days (although four days is optimal to ensure maximum nutritional benefit to the baby)

• Thawed milk in a refrigerator should be used within 24 hours

• Milk stored in a freezer should be ideally used within six months but can be stored for up to 12 months

Tips for storing combined breastmilk

• Wash hands, containers, and breast pumps well to avoid bacterial contamination.

• Always clearly label the container with the first date the breastmilk was expressed.

• Remember to store all breastmilk at the back of the fridge to ensure it is kept at a consistent temperature.

• Always store expressed breastmilk in containers that seal well to ensure that contaminants cannot enter the milk.

While expressing milk can be difficult and frustrating, especially if only small amounts of milk are acquired with each session, it is very beneficial for babies. Combining milk from previous pumping sessions is more convenient and saves space in the refrigerator.  Milk can be safely combined and stored if the guidelines above are correctly followed.