There are several stereotypes utilized in the story, but I think the two best ones are inherent in the character of Mary Maloney.
The first stereotype is the defenceless and emotional pregnant woman. At the beginning of the story she is
She is shocked when her husband says he is planning to leaver her, and as the reader we are appalled that he would suggest such a thing to his pregnant wife who clearly dotes on him.
Mary Maloney did not plan to kill her husband, but once he was dead she staged an elaborate and effective cover up of the crime to protect her unborn child. She becomes driven by the fierce maternal instinct to protect her baby.
As the wife of a detective, she knew what the punishment would be. It made no difference to her. In fact, it would be a relief. On the other hand, what about the baby? What were the laws about murderers with unborn children? Did they wait until the baby was born? What did they do? Mary Maloney did not know and she was not prepared to take a chance.
Her animal instinct for survival is what drives her planning, and her gentle vulnerability is what convinces the police officers that she is an innocent victim. She continues to act as the distressed but dignified wife, and shows hospitality to the policemen who are investigating the crime. However, by getting them to eat the leg of lamb, they are assisting her in disposing of the murder weapon.
Why don't you eat up the lamb in the oven? It'd be a favor to me if you ate it up. Then you can go on with your work.