this is my house with my 5 month old DS - and we are just starting to slow down on the spitting up. He would totally soak himself and me, and even sometimes multiple times a day, large quantities. it would make me wonder how much he was actually digesting. he is also big and thriving, and now we are down to spitting up maybe once a day. I did notice however that we would have a lot of spit up if i ate more than usual amounts of raw onions - we had one weekend that made me call an advice nurse: really large quantities of curdled spit up after every feeding. i remembered i had a lot of raw onions, so it hasn't happened since. not sure if it was related though. i'm thinking once he can sit up on his own the spitting up will slowly dissipate. In the end, if they're a happy spitter with no complaining, it's more of a laundry issue than anything else. i went through burp clothes like crazy. as far as the gagging goes - DS gagged during letdown quite often, until he got a little bigger. i have a forceful letdown and before my milk regulated i had oversupply as well. not sure about the gagging for no apparent reason - maybe look into silent reflux? but if he gags/chokes while feeding, i think it's normal. was for us!
check out these links on spitting up and forceful letdown:
http://kellymom.com/bf/got-milk/supply-worries/fast-letdown/
http://kellymom.com/health/baby-health/reflux/
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