How does Mary’s careful planning influence the outcome the story?

Mary's pregnancy makes it all the more shocking that her husband is planning to leave her. She is six months pregnant—very far along for him to decide to bail out of the marriage. Further, the coldness with which her husband delivers the news must also be shocking. He says to her:

But there needn’t really be any fuss. I hope not anyway. It wouldn’t be very good for my job. She loved her husband up until this point, looking forward to his coming home from work as her "blissful" time. She liked being with him and even enjoyed the way he sat in his chair.

When she finds out he is divorcing her, her initial response is disbelief and denial. She then kills him quickly with the frozen lamb, not even thinking about what she is doing until after he is dead.

Her knowledge of having a child to protect and her desire that it not be orphaned then influences Mary to cover up her crime.

Explanation:hope this helps