The rabbit died

The phrase comes from the early to mid 20th century pregnancy tests which were performed with the use of rabbits, rats and toads. Blood or urine was injected into the animal. Women produce a hormone called human chorionic gonadotropin or HCG after conception. HCG is highly poisonous to the aforementioned animals, and therefore the injection would kill the animal if the donor was pregnant. Hence, "the rabbit died".

In the 1970s, new tests using tube agglutination were introduced. These ring tests used prepackaged red blood cells to detect HCG. However, they were abandoned due to sensitivity to motion. Modern pregnancy tests use monoclonal antibodies to detect HCG. Even minute amounts of HCG can bind to the antibodies, allowing pregnancy to be detected six days after conception. Still, the phrase is used, but the rabbits are bit happier with the way things are now.