While there’s nothing wrong with mixing breast milk and formula in the same container, it’s not recommended simply because you don’t want to waste a single drop of your precious breast milk. Formula from a bottle that your baby has drunk from must be discarded within an hour of preparation. Breast milk, on the other hand, will “keep,” refrigerated, for a number of hours in a bottle that’s been fed. Some mothers mix powdered formula with their breast milk in order to increase the calories their baby gets during a feeding. Please do not do this! Not only does it change the composition of the breast milk, but the micronutrients in the formula will become so much more concentrated that it can be very hard on your baby’s immature kidneys. Always follow the exact directions on the can of powdered or concentrated formula, and never mix formula with anything but distilled water.