lady macbeth takes a very matemal image that of a nursing mother and prevents it why does she use that image in particular? What is that link to her gender?

She has previously asked the "spirits that tend on mortal thoughts" to "unsex me here."  She is planning to encourage Macbeth to murder Duncan so that Macbeth can "catch the nearest way" to the throne.  She knows that he is too "full of the milk of human kindness," so she feels she has to be more manly to help him carry out her plan.  She uses the image of the nursing mother to contrast since she says that she "would dash its brains out" rather than disappoint her husband if she had promised what Macbeth had promised to Lady Macbeth.  This rather violent image reinforces what a "man" would do, someone who is basically emotionless.