An informal, figurative phrase not meant to be taken literally. When Jane said she had a bun in the oven, she meant she's expecting a child, not baking.
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Casual Context
Used in relaxed, personal settings among friends or family, rarely in formal situations. At the family barbecue, Aunt May whispered that she thinks Molly has a bun in the oven.
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Humorous Tone
Often used to add a touch of humor or light-heartedness when discussing pregnancy. Everyone laughed when dad joked that he's getting fat and looks like he has a bun in the oven too.