I think it's probably common that an employed mother's supply will
decrease with time.  I'd check with her about stress, adequate pumping
time and location, and sleep.  I'd also ask about night nursing.  In my
own case, my supply decreased dramatically the one month I tried pumping
instead of nursing my son each time during the day.  We solved this by
increased night nursing (I even fed him when he was asleep) and eventually
rearranging my schedule so I could nurse him at both breaks and lunchtime
with pumping reserved for emergencies.
-- Anne Montgomery, M.D.
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