My friend Melissa has a bun in the oven (she's pregnant).
How to use the phrase 'a bun in the oven' in a sentence
In addition to Mike's example, you could say 'I'm doing some baking; I have two loaves and a bun in the oven at the moment'.
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Please note that a better title would have been A bun in the oven.
Extract from the Posting Guidelines:
'Thread titles should include all or part of the word/phrase being discussed.'