When Macbeth falters in his plan to murder the King Duncan, Lady Macbeth uses this line to force Macbeth into action. She is telling him that if she had made a promise to him, even if it was to kill her newborn baby by bashing his brains out, she would not faulter in her promise. In a way she is telling Macbeth that she is more committed to him than he is to her. She is also telling him that he is weak and less of a man if he is unable to follow through with their plan, when she would be willing to kill her own blood. She is manipulative in her words and this is what forces Macbeth into action.
Have the MacBeth's had children in the past? This quote indicates that Lady MacBeth has had a child in the past, perhaps the fact that their child has died has led the MacBeths to rely on each other a lot, feel depressed or even for Lady MacBeth to become unbalanced. Perhaps she looks to compensate for the loss of her child by ambition for her husband.
In the context of this speech she is showing MacBeth that in stepping back from what she considers he has to do - murder Duncan and become king - he is a coward and is letting her down badly. The image she gives here of the killing of a child is an appalling one, the idea of dashing out the brains of a child deliberately revolts us and turns us against Lady MacBeth. It also shows the sheer force of her character that leads to MacBeth following what she says.
Lady Macbeth emotional blackmails Macbeth”Have plucked my nipple from his boneless gums and dashed the brains out, had I so sworn as you have done to this." She tells Macbeth that had she made a promise, even to destroy her only child at its most innocent and defenceless state she would have. This is influencing Macbeth’s emotions so that he feels he is bound to doing the same, to keep to his word as she says she would sacrifice a part of herself for him.