DD2 (DC3) was born 8wks prem, was on the 50th centile at 4lb birth weight... she will be 12weeks tomorrow. Up until last week she'd dropped and dropped and dropped off the centiles, last week actually falling off the bottom centile line by around 60grams.
She'd been given some formula top-ups between about 8wks and 10weeks and it mucked everything up, she wasn't gaining much weight (despite being on 'catch-up' formula 2 feeds a day) and my milk supply dwindled to nothing. I had a day of feeding her every 10-20mins and she was screaming the whole time she wasn't being fed. The next day my supply had increased enough to settle her and got better by the day. I stopped the formula the day i fed every 10-20mins of my own accord as it'd just caused more harm than good, she put on 140g that week which was fantastic, but then last week only gained 1.5oz (making her 5lb11oz).
Now she's just been weighed this morning and is a staggering 6lb 6.5oz!! thats a gain of 320g (11.5oz) in 8 days!!!
So for anyone else with a slow weight gaining baby i wouldn't worry too much, as long as they are happy and healthy and not loosing weight then dont do anything you dont want to (as in giving formula like i was guilted into)... they will gain soon enough- just give them the time it takes them!!