Hi all
I approached Peter Hartmann with the question of milk synthesis
during a feeding session, and how this affects his measurements of
women's breast volume.
His reply is:

  In
animals as well as women milk ejection results from the contraction of
the myoepithelial cells surrounding the alveoli which causes the
expulsion of milk which was synthesized in the previous between
breastfeed/milking interval.  On the other hand milk synthesis is a
continuous process. However, in women in contrast to dairy animals the
rate of milk synthesis can vary considerably over the course of the
day.  Although I would assume that some milk synthesis occurs during
nursing our CBM(computerised breast measurement)  system cannot
measure  this.  If we compare the
change in breast volume at a nursing with the volume of milk
transferred to the baby we obtain an 'r squared' of about 0.91,
meaning that 91% of the variation in breast volume is due to the
volume of milk transferred to the baby.  Thus only 9% of the variation
is not accounted for - this residual variation could be due to milk
synthesis during nursing, error in the breast measurement system,
error in the measurement of milk intake by the baby as well as changes
in the volume of blood within the mothers breast.  In other words,
synthesis during nursing can only be a very small component of the
milk transferred to the infant at a nursing.  Indeed, if we add up the
rates of synthesis between breastfeeds we arrive at a 24-h milk
production which is very close to the amount of milk transferred to
the baby during nursing (measured by test weighing) over that period.

Unless, mothers in Ithaca breastfeed for very long periods of time at
each breastfeed during the day - milk being consumed at a normal
breastfeed would be milk present in the breast before the start of
that breastfeed. It is possible that the rate of synthesis could
increase during a breastfeed - particularly if the breast was very
full of milk at the beginning of the feed, however, little if any of
this milk would reach the baby at that breastfeed.

Hope this clarifies things!

Fiona Coombes MBBS IBCLC               [log in to unmask]
Nursing Mothers' Association of Australia Breastfeeding Counsellor
Lactation Consultant, Family Physician
Perth Western Australia