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A major physiological fact is overlooked by both these models, however, which is that the baby’s jaws repeatedly compress the nipple-breast/teat complex at its base, at the start of the suck cycle, and do so cyclically throughout the feed. This pressure (approx. 37.5 mm Hg) is likely to occlude the milk ducts with each suck, so it is not appropriate to assume that milk is drawn directly from the breast into the baby’s mouth, on the assumption that the milk ducts remain patent throughout. The milk-filled duct system of the mother’s breast represents a pressure gradient, confluent with the baby’s mouth, which is active at the onset of feeding. Hypothetically, this pressure gradient could ensure continuous movement of milk from within the breast into the baby’s mouth. At the start of the suck cycle, however, with closure of the baby’s jaws, it is no longer active and will only become active again at the end of the suck cycle, when the baby’s jaws reopen, and the teat ducts refill with milk from the breast.

This jaw closure, which causes the pressure gradient to be de-activated, persists for 75–80% of the suck cycle; so the pressure gradient cannot be characterized as being active throughout feeding. This is perhaps the biggest limitation of the two engineering-based models published to date, making them approximate much better to how a mechanical breast pump works. Neither provides a satisfactory theoretical explanation of how breastfeeding works (or of manual breast expression for that matter). Further concern should be raised over the assumption that the milk duct apertures remain open throughout; this is unlikely given the close approximation of the nipple to the soft tissues of the baby’s mouth, and the high suction pressures generated within the oral cavity.