A bun in the oven
What's the meaning of the phrase 'A bun in the oven'?
Pregnant. Oven = womb, baby = bun.
What's the origin of the phrase 'A bun in the oven'?
This phrase might have been derived as an allusion to the loose connection between 'baby' and 'bunny'. However, it's more likely that this, and other terms for pregnant, like 'up the duff' and 'in the pudding club', all share the same euphemistic source.
The phrase originated in the 20th century. The first citation I can find is in Nicholas Monsarrat's Cruel Sea, 1951:
"'I bet you left a bun in the oven, both of you,' said Bennett thickly... Lockhart explained ... the reference to pregnancy."