My LO took 22 days to regain his birth weight, but is still on the 0.4th centile, but following the curve 'nicely' & is gaining, so our Dr isnt too worried.
That being said, between weeks 4 & 5, I supplemented once a day with a 3oz bottle of Aptimal formula...when doing this I expressed to keep my supply up. LO never had any issues taking the bottle or anything luckily. I stopped giving formula after this though, as I didnt feel comfortable with it, so instead, I started feeding him every 2 hours & hand expressed off the first lot of milk during the night so he was getting more hindmilk rather than tons of foremilk & also used compression feeding to encourage him to feed more when he was really sleepy. I now make sure I feed him from one side, then burp him, change him (even if he doesnt really need it) then burp him again as this makes him more awake, then I feed him again from that same side to increase the likelihood of him getting more of the fattier hindmilk... then if he comes off still awake, then I let him have the second boob, and he has 1 4oz bottle of expressed milk before bed every night too.
That's worked for us, but I would definitely advise speaking to your HV/GP about your concerns & see what they think, as they might have more helpful suggestions.
Does your LO have at least 6-8 wet nappies a day and also at least 2 dirty nappies a day? (sign LO is gettin enough) Is the fontanelle sunken at all & if you pinch her, does her skin pinken up again quickly (signs of dehydration or not)? Is she reaching other milestones & does she appear happy in herself?
(I know a friend of mine added 1 spoon of formula to her EBM each day to make it richer & help her LO gain weight, but I dont know if it worked or not as I havent seen her for ages. I also dont know if that's a good thing to do or not, so dont do it until you speak to your hv/gp, ok?)
Giving the odd bottle of formula is definitely NOT giving in; we all have to do what's best for our babies!