In "Lamb to the Slaughter," how does Mary react to her husband when he says he doesn't want to go out?

This is a critical moment in the story as it marks a
transition from what we expect to be a happy story focussing on the love of Mary Maloney
for her husband to something much more interesting. Reading the text carefully tells us
that normally on a Thursday night they go out to dinner, which is why Mary Maloney has
not prepared anything for her husband to eat. However, even when she says that she could
prepare something quickly for him if that is what he would like, he says that he is not
hungry at all and asks her to sit down. Note how the narrator describes her
feelings:

It
wan't till then that she began to get
frightened.

Of course, this
is when he decides to tell her about his decision to leave her at this particular
moment, even though she is pregnant. Thus the mood of the story changes suddenly and we
see Mary Maloney for the first time getting scared.