“bun in the oven”不是指“烤箱里的小圆面包”，引申为怀孕。

【例句】
John, I've something to tell you. I've a bun in the oven.
约翰，我有事要告诉你，我怀孕了。

She had a bun in the oven. But she wanted to surprise her husband by not telling him right away.
她怀孕了，但是她想先不让丈夫知道，她想给丈夫一个惊喜。