When I had ds I wasn't the walking type but now I am and would like to get a backpack for dd, who is 5 months old. Do they have to be sitting up before you can use one or is it based on weight or doesn't it matter? She isn't sitting up yet, although she's close to it and she's a big fat baby so although I've no idea what she weighs I'd say it's well over a stone. TIA to anyone who knows. I did look at the mumsnet review section but although most of them say from six months I couldn't work out why.