Tina,
I beleive there is research evidence that children grow in spurts.  Therefore,
a study that took weights only twice or three times could easily miss spurts
in weight gain.  What mother hasn't put her kid's shoes or clothes on one
morning and suddenly they didn't fit, when they fit the day before?  To be
valid, a study like this should take daily weights and generate a curve for
each baby.
        Another problem with the drive to quantize is that unusual does not
necessarily mean abnormal.  The reason normal processes were first charted was
to find the tails of the bell shaped curve, so that persons who fell in the
upper or lower SD could be scrutinized TO SEE IF THERE WERE SOMETHING WRONG.
Look what happened with labor progress- only normal laboring women were
studied, then their average rate of dilation became the 'normal' rate of
dilation!  This is a major contributer to the US's sky high c/s rate.  We have
done the same thing with growth charts.  They were meant to tag the slowest
and fastest growing children so they could be examined for cretinism,
gigantism, dwarfism, and ftt.  Being at the bottom of the chart is not a
guarantee that a child is not thriving, it is a way to screen for children who
should be tested just in case....  (Our NYC public schools have an incredible
variety of shapes and sizes of healthy children, there are kindergarteners
smaller than my kids were at 2 years old, there are third graders bigger than
me.)
        We've also done this with alpha fetoprotein screens.  Women think it
is a test, whereas it is simply a screen to seperate fetuses into higher and
lower risk groups.  The higher risk groups go on to be tested.  Growth charts
are a screen.  Being on the tails of the bell shaped curve merits further
scrutiny, but is not in and of itself abnormal.
        Take home message:  statistically average is not the same thing as
normal.
Catherine Watson Genna, IBCLC