Usually when they’re falling asleep feeding, taking forever, fussing, seeming uninterested you will know they’re ready to move up x
So I asked my midwife this and she said it’s all aload of crap they can stay on size 1 all the time but some babies do get frustrated because of the flow they will suck and cry whilst drinking of choke and cough lots trying to drink and suck quickly but she said they don’t need to move up sizes same is they don’t have to come off of breast milk formula and onto stages formula.
We moved to size 2 and it seems to drown her mostly. So moving back to 1. Talking to other mums at a class the other day they have experienced similar when moving up a teat.
You can definitely move to size two and try it out as it’s faster flow for baby, however if he’s not fussing, finishing the bottle and seems content no need to switch if you aren’t wanting too x