I'll take a swing at this.

It doesn't matter if you are already nursing or not- the milk after delivery will 
be the composition of colostrum. The changes in the milk composition after 
delivery are happening on a continuum that has everything to do with the 
paracellular permeability from pregnancy to lactation. 

The space in between the cells of mammary gland is permeable enough to 
allow sodium, chloride, and lactose to cross in between cells during pregnancy 
and immediately after birth.  (I assume as well that it causes the changes in 
the taste of the milk as pregnancy progresses.)  After removal of the 
placenta, those paracellular tight junctions close over a few days.  When that 
happens, sodium and chloride decrease in the milk and the lactose 
concentration increases (because those substances are being regulated by 
specific transport pathways, not just flowing in between the cells)  Water 
follows sugar and the milk increases in volume.

So...the presence of the placenta and it's high levels of progesterone makes 
the cells more permeable as pregnancy progresses (regardless of the state of 
lactation) allowing for the higher sodium and chloride and lower lactose 
associated with colostrum.  After the placenta is delivered, the tight junctions 
close over the following days and we have a different recipe, not colostrum, 
being produced.

And because it's placental progesterone that affects pregnancy milk volume 
and release, placental delivery can only help your "stores of colostrum."

A good lactation physiology article should help the midwife (if you want to 
fight this battle.)

Cheers,

Jenny Thomas
www.drjen4kids.com

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