Quoting Baby Liam =D:" I just didn't know if I was supposed to get slow or medium. He has never had formula. Breastmilk only."Is he going to be drinking formula or bm through the bottle?? Formula is thicker than bm so you might want to go fast flow. If it's bm you could do medium. However, at 10 months old he likely has a good enough suck that it really doesn't matter. As for the bottle, hopefully he will just take it because you are not around to offer the boob.