I am speaking from "clinical experience" here and do not have hard
data, but I do not think that return of menses is associated with a
sudden decrease in milk supply, at least in the usual situation.  My
wife nursed 3 children at least 3 years each, and there was never a
change with menses (once she got them back, which was 14 months after
the babies were born).

You can read in books, and I think in LLLI literature, that there may
be a decreased milk supply during periods.

However, I hear from a significant number of women experience a sudden
decrease in milk supply around 3 or 4 months after the baby's birth.
Many of these mothers have had very good milk supplies, and no
problems up to then, and no return of periods.  I have no idea why.  A
short course (2-3 weeks) of domperidone usually bounces the milk
supply back, and the mother and baby are back to normal.

Often the milk supply is still quite good, but babies like *flow*.
When milk flow is good, they are happy.  When it slows down, even if
they have not had enough, they sleep at the breast if they are young
(less than 4 to 6 weeks), and then they pull away and are fussy and
unhappy.  They often, but not necessarily, gain well, but are not
happy, feeding frequently and for short periods.

Maybe it was a coincidence, maybe a true thing.

Jack Newman, MD, FRCPC