Do not fill the container completely because breast milk expands as it freezes. (babygearlab.com)
Once this system breaks down and the animals freeze solid, the ice tears up their insides: The water in their cells expands as it freezes, shredding membranes and dislodging organelles. (discovermagazine.com)
Breast milk expands as it freezes, so don't fill containers to the brim. (mayoclinic.org)