Nofia,
Speaking anecdotally, I've had three moms who were well-established in
breastfeeding when they suffered milk supply faltering (a few months
postpartum).  I covered all the bases with them in terms of what things
may have changed, previous feeding frequency history, etc, and even had
them try frequency nursing to try and bring milk supply back up.  Finally
it came down to the mini-pill being the only real change. Each of these
mothers abandoned the mini-pill, and each of them then experienced a
rebound in supply.

I know the research tends to deny this--- and maybe, by the statistics,
it doesn't occur often enough. However, we are the ones who get the calls
from the moms when something goes wrong, not the docs. And, I do wonder
how many women suffer from reduced milk supply and never ask for help.
We all know that there are so many perceptions about the overall
vulnerability of milk sufficiency that I can easily picture women
assuming that what is happening to them is normal.

I did some research on this topic for a conference presentation, and came
up with little other than the "fine after 6 weeks postpartum."
Personally, I prefer that mothers wait at least 3 mos, until lactation is
better established and entering the autocrine control stage, than risk
messing things up during the endocrine control phase.

-Lisa Marasco, BA, IBCLC