Do you pump? If you do, the easiest way to transition is to mix breast and whole milk together in one bottle for awhile. When we started, the bottle would be about 75% BM and only 25% WM. Then, we gradually shifted to 50/50, then 25/75, until it was 100% WM. Or sippy cup instead of a bottle, of course - whatever you choose.
Don't rush the process too much. Cut out about one nursing session per week and replace it with a bottle, starting with whichever session is least important to her routine. For example, I cut out post-nap nursing, then pre-nap nursing, then bedtime, then first thing in the a.m. I did once ever two weeks or so, but you can do once a week and still be done in about a month. You need it to be somewhat gradual for the process to go more smoothly and for her to put up less of a fight.
At this age, she really doesn't need to be nursing at night anymore either (unless you mean bedtime, not middle of the night). Or unless she is really tiny.
Good luck!
K.
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