Freezing & thawing breast milk

Lauralee

I have been pumping then freezing my breast milk. I use my Dr brown bottles to measure how much I am putting in each bag & mark the amount on it. I thaw them in the fridge and warm them up in water. When I pour the milk into a bottle it is no longer the amount I wrote on the bag. Instead of 5.5oz it will be 5oz. Is anyone else having this issue? Is this a normal thing or am I doing something wrong??