Is expressed human milk sterile?

• A. Yes
• B. No
• C. It depends on handling
• D. Only when stored cold

Answer: (B)

Expressed human milk is not sterile. Bacteria are commonly present in human milk as part of the natural mammary microbiome, and microbes can be introduced during expression, handling, and storage via skin, pumping equipment, and containers. Cooling or freezing slows growth but does not sterilize the milk, and even donor milk that is pasteurized is not guaranteed to be sterile—pasteurization reduces pathogens, but post-pasteurization contamination can occur if handling isn't sterile.