I have been wondering how to do this too. I am working on a baby cardigan and it's too big. I started with 44 stitches medium size and small in the pattern is 40 sts so I may be wrong but my thoughts are that if I go down in my co's by multiples of 4 and the same with the other pieces I will come out with the size I need. I am CO 32 st's and seeing if that will work.
Maybe you could do the same but increase by whatever multiples in your pattern calls for. Example: 0-3 months 33 st, 3-6 36st as you can see your CO 3 st more as you are going up in size.
Do you think this could work to obtain the size's that we need?