LO flattens/squishes bottle nipple during feeding...next size up?

beachgirl24

DH says he sucks so hard that he ends up squishing/flattening the nipple and DH has to take it out for a minute and then give it back to him after it's reinflated itself, so to speak.  Does this mean its time for the next step up in nipple flow?  Or he's just sucking really hard?  How do I know when it's time to move up? We use the Medela widebase slow flow 0-4 month nipples.  DS is 12 weeks.  TIA!