Your baby can literally change sizes over night. Even if your baby is a few pounds away from stepping up to the next size, have at least a small package around…you’re going to need them eventually anyway.
You will reach a time when you will have constant blow outs…either up the back or out the legs.
Unfortunately, if you’re experiencing blowouts up the back, you can go up a size, and it will come out of their legs…the only option you have at this point is to try switching brands for a different shape or waiting it out and having lots of extra outfits ready.
(This applies mostly to Preemie, Newborn, Size 1 and Size 2… once you get into size 3, you’re usually good for about a 10 pound window of their life)