[Pansy-Mae]

Sophie,
There really is no mystery here at all.
The simple answer is that it is progesterone and prolactin preparing the breasts to produce milk. Simple.
Of course that is only a part of the wholesale changes to hormone levels throughout the female body during pregnancy, but that is the bottom line to your question.
If you look through Isabelle's thread she lists all the herbs and their effects ot do exactly what you are suggesting.