Dear Jan, a newborn can't be failure to thrive in first few days or so.  As
you said baby has to have some sort of a track record, something to be
failing (and no, a newborn who loses 7%-10% of birth weight is not failing
to thrive - s/he is just getting off to a slow start, and yes, it is a red
flag.  Check suck, positioning, milk transfer/production and so on).  The
formal definition of FTT is (1) dropping 2 or more standard deviations in 2
mo. (2) weight below 3rd percentile or (3) 2 standard deviations below the
median.  However, FTT is NOT a diagnosis, it is a cluster of symptoms.  It
can be organic, non-organic and mixed.  I have had several "mimi babies" in
my practice who grow consistently and steadily  on the the 2% or 3 % line.
they are petite, not FTT!  You have to remember to look at the parents.  As
one of my nurses' aides says "you don't get an elephant from an ant.!"
So anyway the baby who gets off to a slow start, dropping a tad too  much
and then regaining, isn't FTT by any stretch of the facts or definition.
Sincerely, Pat in SNJ