I think they will let you know hun I have been using a back carry from about 3 months in the MT and ergo and if Ashton isn't happy he will tell me with crying/whinging or by the way he is thrashing about.
I like having him on my back though, with a 3 and 5 year old it's handy not having something attached to the front of me... pregnancy was bad enough lol Plus he usually goes to sleep back there