My 4 month old went from 11 lbs 13 oz at her 2 month check up to 11 lbs 3 oz at her 4 month check up. She seems completely healthy although my pediatrician is running a metabolic panel just to be safe.
We are all puzzled by the loss. It doesn't seem to be a supply issue. She latches well, always seems full when done, and has 5-6 wet diapers a day. She doesn't fuss when she is done nursing. Isn't lethargic, etc. She nurses (on average) every 2-3 hours during the day, and she sleeps 6-7 hour stretches at night.
I did have to give up dairy for a few weeks, because she only has 1 bowel movement or so a week. The doctor wanted to see if it was a reaction. He has given me the go ahead to slowly work it back in. She only is a once-a-weeker whether I eat dairy or not. I just started adding it back in yesterday.
She was 7 lbs 10 oz and 21" when she was born. She was 7 lbs 6 oz when she left the hospital.
She is now 4 months old, 11 lbs 3 oz, and a whopping 28.5".
The pediatrician doesn't seem overly concerned. He told me to just try to get her to nurse more and bring her back in 2 weeks for a weight check. He said if she gains anything by then we are good.
We do live in Atlanta where it is pretty hot, and yesterday before her appointment I had both kids at the Botanical Gardens. She had only nursed ONCE prior to her appointment, and that was 6 hours before it. So I think a part of it could be she was just dehydrated slightly.
Edited to add: I nursed my 2 year old for 18 months without supplementing or having any issues. He is also very tall and skinny. He's 39" and weighs 28 lbs. He has always been like that, but he didn't lose weight until he was a year old.