Mixing Old and New Milk...

Once you "marry" product the expiration date becomes that of the "old" product so keep that in mind. As well, I just would not personally.

I combine bags all the time. My son will be 6 months tomorrow. I pump at work. When I get home I combine all of my pumped milk into one bag to freeze. Sometimes it's 4 oz and sometimes it's 6. My son is taking 8 oz bottles now, so I will take 2 4oz bags and thaw them and mix them together. I date my bags so they are usually just a day or 2 apart. use your older milk first.

if you're going to use all of it right away I don't see a problem with mixing them. Otherwise I would keep them separated and watch the expiration date on the older one.

My baby is considerably older, but this is how I do it: I pump every day at work. At the end of the day, I put one full bottle in the fridge for the next day and add what's left to whatever is in the freezer. (I add one to three ounces at a time.) Then, on Friday, I put what I've pumped for the day into the freezer for the weekend and take out on Monday morning (for Monday consumption) the stock that I've been adding to, leaving the freshest milk in the freezer. If that stock fills before the weekend, then I go ahead and use it and put the freshest batch into the freezer.

I guess my answer is that I think that you're okay to do that for a few days, but no longer. Keep in mind that, not only is there older milk in there to consider, but also that you keep opening and closing the bag/bottle, affecting the freshness. Once you close it, you should try to open it as few times as possible before you are ready to use it.

Also, your milk changes with your baby's needs, so I would think that at this stage you shouldn't save for more than a week or two at a time, either. (Was that even part of your question?)

Congratulations on that sweet baby!! Enjoy!