Sister,
I had something similar to this going on with my MIL. It wasn't about feeding, it was just that she would "take over" whenever she came, almost as if she needed to take care of my son because I obviously couldn't do it. I finally resolved it by just sitting her down and telling her "this is OUR child, and we are perfectly capable of raising him", etc. She actually apologized and told me she didn't realize she had been doing it, it was just instinct.

I would explain to your grandmother that some babies only eat a little at a time, which requires them to eat more often. But supplementing your baby if your doctor doesn't think it's needed can actually cause more problems. Your baby could not only have more problems with gas, but her iron levels could get too high, since most formulas are made high in iron to support a strictly formula fed child. Just my two cents. Good luck.

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Tigriz
Mom to Alex - 1/21/06