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