Just following along as I am going through a similar situation. I took J to md for a prescription repeat and he did gain over 2 lbs in 2 months, but he dropped from 70 to 58 percentile in weight (he is over 20 lbs and is quite tall as well - 98% for this). My md is new and sometimes she says things without thinking and it really concerned me. She finally said it is probably because he is so active now, but I still worry and am trying to come up with foods DS will enjoy. He is not so into purees right now, he wants to be independant and do it himself. Now I have to take him back every month to be weighed.
A friend of mine, her son did have a hard time with weight gain and she went to a dietcian who recommended butter as an excellent source of calories and 'good fat'. She recommended putting it on pretty much everything - and it did help. DS has mpsi so we can't do this
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Protein and carbs are also excellent sources of calories. I love bread and can attest to this
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Good luck, and don't worry about C's weight. Was she a lot taller at this check as well? I know a lot of times LOs grow then catch up with weight.