I just wanted to say [and not in any way competitively] my DD weighed 5lb 13oz at birth and gained well, also for the first 8 weeks. She'd climbed a percentile, but then she dropped down two and is now on the bottom line with a small gain weekly. She's now 18 weeks and weighs 9lb 14oz. She's still gaining, just slowly.
I am getting the opposite advice from you and not to wean just to help improve on weight gain, as they are still mainly getting their calories from Breast or Formula, they would only be taking less than an oz to start with and will not improve immediately to warrant weaning.
My family have made me feel like I am starving my baby. My Gran insists I feed her carnation milk, as that's what she had to do for all her kids as they were slow gaining weight. I've spoken to a few other parents who have babies growing similar rates and they've all done different things. I've taken their info on board, done a bit of research and going to persist with the BF/FF for now. Do you have a feeding group at your local children's centre, it's been a help to me, just to hear other parents tales and to absorb their experiences.
My SiL was told to change from BF to FF as my neice will gain more from FF, this turned out to be old tosh and she continued gaining on the same percentile as before and she regretted her decision. The advice at the time was to wean at 4 months, but she held off until DD was grabbing and trying to munch their food [BLW] which was a little over 5 months.
Remember that you don't have to follow what they say, try and discuss it with them, if it helps write down your questions before you go, and take a pen to jot down their answers and any good sources of information they recommend.
I am going to wait until DD shows signs definate signs of weaning. She can sit up well and has pinched a carrot off my plate before and waggled it about and sucked it, but has yet to give it a munch.
I think sucking is something they all do and isn't a sign of weaning. DD will suck anything and has since she was 9 weeks. I think it's an exploration thing more than anything.
If she can sit well and not supported, slumping makes eating a nightmare for them, poor digestion etc. If she loses the tongue thrust, if you put in some food and it mostly comes back out with her tongue thrusting out then she isn't ready, try again in a few more weeks. You can try putting some safe food within reach of her paws and seeing what she will do. A cooked and cooled carrot stick would be a good example, or a broccoli floret. Make sure it's cool though and not covered in gravy/salt/pepper etc.
It's not easy knowing what to do with so much opinion about, I felt like a failure for quite a while and now starting to get a bit more upbeat. I've cut down on weigh in's twice a week. DD is happy, bright, active and inquisitive. I'd certainly know if she wasn't feeling well and would seek help, but she's gaining slowly but surely.
Sorry for long post. K.