You should not use microwave oven to thaw and warm the breast milk. In the case of a microwave oven, a microwave goes through the breast milk, which may breakdown the important nutrients contained in the milk. The breast milk has to be thawed gradually and given after warming to the appropriate temperature.   It is, also, considered that frozen breast milk cannot be called breast milk even if a microwave oven is not used. The fat and nutritional components certainly change by freezing and thawing. Therefore, the nutritive value and other components are unavoidably less than those in the breast milk sucked directly by the baby.