Before Patrick announces to the six-months-pregnant Mary that he is planning to leave her, Dahl foreshadows this moment by having Patrick repeatedly decline Mary’s repeated offer to prepare food for him, as seen in the following passage:
"Darling," she said. "Would you like me to get you some cheese? I haven't made any supper because it's Thursday."
"No," he said.
"If you're too tired to eat out," she went on, "it's still not too late. There's plenty of meat and stuff in the freezer, and you can have it right here and not even move out of the chair."
Her eyes waited on him for an answer, a smile, a little nod, but he made no sign.
"Anyway," she went on, "I'll get you some cheese and crackers first."
"I don't want it," he said.
Here, Mary waits on Patrick after he returns home from work, asking three times if she can prepare food for him. Rather than saying yes, he says “no,” remains silent, and then says, “I don’t want it.” The fact that Mary “wait[s] on him for an answer, a smile, a little nod,” suggests that this sort of coldness is abnormal behavior for Patrick, foreshadowing Patrick's big reveal to come. As preparing food for her husband is one of a housewife’s primary domestic duties, it is clear here that Patrick’s rejection of the food is a rejection of Mary herself.