Yeah they recommend keeping the teat size at the lowest one possible if still breastfeeding x
My little boy moved to size 2 teats for his expressed milk as he started fussing on the size 1. It's not the size of the teat itself that changes, but the flow. He is able to almost empty my boob in 5 minutes now compared to 15-20 minutes a couple of months ago so I guess he is more efficient at getting the milk out so wants a faster flow on the bottle. I tested it by having him feed from one boob til he was done, pumping the other side while he fed for about 5 mins. When he was done, i attached a pump to the boob he had been on until the other side was finished pumping (20 mins or so). The empty boob only got less than an ounce into the pump while the other had almost 5 ounces. So he must have had about 4 ounces, maybe more as he had fed from the other one during his last feed so it would have been pretty full.
My baby is 12 weeks and we use MAM bottles teat size 0. We tried size 2 but its too big and she wont take it. They recommend using size 0-1, which have slow flow so I have ordered size 1 teats for when my baby is ready.
As far as I know you keep the teat and size the same as you need slow flow. Especially as it’s only the one bottle, it needs to mimic the breast. But you should replace them every 3 months is the advice x