This worked for the first 5 months with my baby boy...than he grew out of the onesie at 6 months. He is a very tall boy for his age. But it was still fun to see him grow so fast.
Here is the collage I made for him out of his monthly pictures in the onesie:
I know there are a lot of new mother's out there and I thought I'd share this fun idea. I am starting to do this with my new baby girl, go figure she was born a lot smaller than my boy was and I bought an 18 month onsie just to be safe - I guess we will see if it works out!
And you can start the documentation at any stage - if you have a month old just start with the one month picture and document until 12 months.