I have been trying to switch to the bigger size nipples, but DD just isn't having it! So I suppose you can try whenever you feel DC is ready, I think they let you know if they aren't! Then again, DD isn't a fan of the larger bottles either (she apparently wants two smaller ones - yeah, that sucks!) so I still haven't come up with a good solution yet!
We use the bigger bottles sometimes (5-6 oz) but haven't gone up to the next level because I'm BF and DD is just fine with the slower ones. Take your cues from your DC. If they start to get frustrated, try going up a level. You just need to replace them when they start to get worn...
Re: few ? about bottle nipples
We switched to a faster flow when we noticed DD was tiring herself out with feedings. We tried a faster nipple and she started eating so much better!
Replacing? I'm curious to see too. I've just been sterilizing them like crazy.
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