Wow, never thought this question would come up, but I have done it twice with complete success and did not have to 'pump and dump' as some people offered up below! I wasn’t about to invest all that time and energy pumping and just toss it down the drain, but to-each-their own!!! Although luggage restrictions weren’t in effect like they are now when I did this, here is what I did.
I carried an extra suitcase with a medium styrofoam container. Depending on your milk supply/flow, a small to medium cooler will work too. I used the bags to store my milk in the fridge, NOT the freezer compartment, while in the hotel. The reason for this is because once you freeze and allow the milk to thaw, you have to use right away...But if you slightly delay freezing until you are ready to head back home, you will be fine. The night before leaving I went to a local big chain grocery store, purchased a BIG slab of dry ice (more readily available than you would think!!!), went back to the hotel and packed the styrofoam container with the refrigerated milk with the dry ice placed on the top and bottom (I broke it in half). By the morning, most of the milk had already started to freeze and the milk bags that had not, I just rearranged, moving them closer to what was left of the dry ice (very little remained). Heading towards the airport, I stopped back at the grocery store to pick up another slab, take note of the weight this time, add to the cooler and you are on your way! The airlines have a policy listed on their website containing information on dry ice. As long as you declare it at check in, they allow you to carry it on. This was taken directly from the Delta website so make sure to check with the airlines you will be traveling with! --- Dry Ice, Delta will accept packages containing 4.4 lbs. (2 kgs) or less of dry ice when used to cool non-hazardous perishables in carry-on or checked baggage.
As soon as I got home, I unloaded the container that had 25-30 full bags (can't imagine dumping all this!), all milk was completely frozen since I had added another slab of dry ice before leaving, and placed it in the freezer/deep freezer! Repeated on another business trip I had to go on and everything worked like a charm.
Now, with all the luggage restrictions and extra fees/charges that seems to be on almost every airline, you might want to get a cooler when you get to your destination and just simply use FedEx on your return trip to get the milk back home. Check the cost of each that way you are informed.
FYI - Since you will be transporting a breast pump with you, just be prepared when going through security that TSA will look hard at the pump assembly, the mechanical piece. They will look in the carry bag and they will swab it for explosives. They did this each time I traveled. Don’t freak out, this is standard and just a precaution for the world we live in today! Just be sure to let them know it is simply a breast pump since you are still nursing mom. They still had to do their job, but they were a little more understanding and really tried to keep their hand off the stuff that needed to remain sterile!
Best of luck…You can do this!!! Just remember to keep up your ‘schedule’ while gone and get your pumps in regardless of what is going on!