diaper size question

flamencogal

I recently moved Nate up to size 2 diapers but now I'm wondering if we need to go to size 3.  The past 2-3 days all of his poops go up his back and out the leg holes.  Twice I was holding him when he pooped and changed him almost right away and still poop up the back and almost out the legs--would've gone out the legs if I hadn't gotten him to the changing table right away.  The diapers seem to be holding pee just fine.

He's 3 months (today!) and probably about 13-14 pounds.  We use Seventh Generation diapers and have never had a problem with them not being able to hold enough.  Since Scott was a preemie I have no idea what "most" infants Nate's size are wearing diaper-wise. Thoughts?