There's a difference in cells between when they are immature and when 
they have terminally differentiated. In the less mature cells in the 
developing breast during pregnancy, progresterone receptors develop in 
the cells that are going to be ductal cells. In mature lactocytes, the 
progesterone receptors are downregulated in the process of maturation a 
few days before the impending birth. This strengthens the hormonal 
signal when the placenta comes out, there is suddenly less progesterone 
in the system, and fewer receptors to boot.

Hope this makes sense.
Catherine Watson Genna, IBCLC  NYC  tired in NH

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