Experts have different opinions on this one. Some say you shouldn't leave milk in the fridge for more than six hours before freezing, others say 24 to 48 hours, and still others say it's fine to freeze milk that's been refrigerated up to eight days, provided that baby is healthy and the milk is for home (not hospital) use. Bacteria doesn't seem to be a big problem, since one study showed that milk left in the fridge for eight days actually contained less bacteria than freshly expressed milk. (Go figure.) Still, it can be reasoned that the milk might spoil faster or be less nutritious when thawed than milk that was frozen immediately after expressing.
Our best advice: If you're planning on freezing your milk, go ahead and do it ASAP. If you need to freeze milk that has been sitting in the fridge, give it a sniff test (to make sure it's still good) before freezing. Be sure to store it in clean, well-sealed glass or hard BPA-free plastic containers or freezer bags specially designed for storing breast milk. Add a label noting the date the milk was expressed, and consider adding an extra note if the milk stayed in the fridge for a while. (This will remind you to do another sniff/taste test before offering the milk to baby…just in case.)
If baby is sick, preterm, hospitalized, or otherwise at risk for illness, freeze any refrigerated milk within 24 to 48 hours. And remember that fresh milk (straight from the breast) is the healthiest for baby; it is most nutritious and contains the most anti-infective properties. Refrigerated milk is next in line, followed by the frozen stuff.