Right now I BF & pump. When we run errands, go out to eat, etc. we always bring a bottle, but it's not always easy keeping it cold then being able to warm it up quickly. I've considered using formula on these occassions because everything can be kept @ room temp but not sure how DD will do w/ formula or if it's a bad idea to use it so sporatically.
Re: can I supplement formula occassionally?
You can but if you don't pump to make it up, your supply will likely drop.
Not to be pushy but why don't you just breastfeed her when you are out?
Breastfeeding Counselor with Breastfeeding USA
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If you are only going to be out and about for a few hours, there is no reason to keep the breastmilk cold. It will be fine. Otherwise, you could bring along a thermos of hot/warm water to stick the bottle into when you're ready to use it. I personally would not want to stick the occassional bottle of formula in for any reason unless it was for some kind of medical necessity, but that's just me. Formula is different on the tummy than breastmilk.