What is the origin of the idiom "have a bun in the oven" and when to use it?

The origin of the phrase "have a bun in the oven" is believed to come from early 20th-century American slang. The expression draws a metaphorical comparison between a developing baby in a woman's womb and a bun being baked or cooked in an oven. This phrase is used informally to talk about someone's pregnancy, often in a lighthearted or playful manner. It is commonly used among friends, family members, or in casual conversations.