Get it? Garden-variety? Rabbit?
If the woman’s urine contained hCG (pregnancy hormone), the rabbit’s ovaries would react by producing little pockets of blood. (These were harmless to the rabbit.) Unfortunately, this was in the day before see-through rabbits, so the only way to see the ovaries was to kill the rabbit, cut her open, and peek inside. So, the rabbit actually died whether or not the woman was pregnant. Somewhere along the line that little detail was lost, however, and “the rabbit died” became slang for “being pregnant.
