No harm in starting now, Follow the sizing on the patterns & they are bound to fit at some stage.
Im making stuff for my unborn. I am also making tuf for my DD in unisex/boy colours so Mr Doodle will get wear of the also as they will share the same month of birth so season/size wise should be similar.

Im not sure if prefolds & soakers/longies will be your best option. You may find (from what I have heard/read, haven't actually done a NB in cloth yet) that you will want some good fitting covers for your prefolds. Nice snug fits around the legs to help prevent leaks. But jump into the cloth thread for more on that side of MCNing