Phrase used to indicate that a woman is pregnant. The term originates from an early pregnancy test. It was discovered that when a pregnant woman's urine was injected into a female rabbit, the hormonal difference would result in detectable changes in the rabbit's ovaries. Although this form of testing was always fatal to the rabbit, no matter what the results were, the public somehow got the misconception that only a positive result killed the rabbit.
"We'll have to get married. The rabbit died."