Flat! I have one of those The First Years BM storage thingys that freeze them upright and flat. It makes them very uniform, and I then transfer them to my deep freezer laid sideways in one of those $1 plastic shoeboxes from Walmart.
I lay them flat in my fridge freezer, but store them upright in boxes in the chest freezer. I usually wait until I have 12-15 of them in the fridge freezer and transfer them all at once.
Flat - more compact to store and thaws quicker. I'm not sure if you are already doing it, but then you can put all your small bags into a large freezer bag to make it even more easy to store and protect from freezer burn too.
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Re: freeze bm flat or upright?
Aiden 10.17.07 Emma 07.15.10
I lay them flat in my fridge freezer, but store them upright in boxes in the chest freezer. I usually wait until I have 12-15 of them in the fridge freezer and transfer them all at once.