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One final piece of unique evidence comes from a historical study, not from any recent research. In the 1980s, Alan Lucas, then based at the John Radcliffe Hospital in Oxford, came up with the novel idea of measuring milk transfer from a mother to her baby directly, by placing a flow transducer between them, housed in the tip of a latex nipple shield [25]. The research team (Bio-Engineering Unit) developed a Doppler ultrasound flow transducer which insonated an area of parallel milk flow, created as breast milk passed through the transducer body. This technique provides completely unique views of instantaneous milk transfer during suckling [26]; I have been able to revisit a proportion of the original milk flow traces, undertaking some fresh analysis of them, in an attempt to resolve some the issues emerging from the “revised suckling physiology” above.

If, as Geddes et al. [11] assert, added suction (ETDs) is the predominant force in milk removal from the breast, then one would be likely to observe a “mid-suck” peak in milk flow, with relatively little milk flow either side of this. In practice, this is not the case – peak milk flow is invariably seen early in the suck cycle (first 20%), tailing off towards the end (Fig. 9). In many sucks, following an early high amplitude flow, a later more attenuated phase of irregular milk flow may be observed; this is most conspicuous in sucks of longer duration. These sucks are most likely to be those which include an ETD, which were shown by Eishima [7] to extend the suck duration.

More commonly, the two phases of milk flow grade into each other, so first we have a high-amplitude, short-duration flow, followed by a lower-amplitude, longer-duration flow. The net contribution each of these makes to milk transfer may be the same, although it is important to remember that the secondary peak of milk flow may be absent in a large proportion of sucks. This novel source of information about milk flow during suckling suggests that baseline suction and PTMs are uniquely responsible for initiating and maintaining milk flow on each and every suck. When ETDs are superimposed on the incipient rhythm, they appear to enhance milk flow, mainly by sustaining it over a longer duration.

Fig. 9. Section of a milk flow trace obtained with a Doppler ultrasound flow transducer. Suck duration is shown in the box around each one, so, in terms of whether they are long (L) or short (S), this series of seven sucks is L-S-S-S-L-S-S.

This unique insight into the process of milk transfer has been provided, not by new engineering-based models, but by a much earlier piece of research. Nonetheless, we have only recently been able to explain fully the complex shape of the milk flow profile in light of the evidence that both PTMs and ETDs coexist during breastfeeding, demonstrating that the baby both suckles and sucks milk from the breast.