Technically, ALL the rabbits died...but the phrase was used anyway.
The original test actually used mice,[SUP]
[2][/SUP] and was based upon the observation that when
urine from a woman in the early months of
pregnancy is injected into immature female
mice, the
ovaries of the mice enlarge and show
follicular maturation. The test was considered reliable, with an error rate of less than 2%.[SUP]
[3][/SUP] The rabbit test consisted of injecting the tested woman's
urine into a female
rabbit, then examining the rabbit's
ovaries a few days later, which would change in response to a
hormone only secreted by pregnant women. The hormone,
human chorionic gonadotropin (hCG), is produced during pregnancy and indicates the presence of a fertilized egg; it can be found in a pregnant woman's urine and blood. The rabbit test became a widely used
bioassay (animal-based test) to test for pregnancy. The term "rabbit test" was first recorded in 1949 but became a common phrase in the English language.[SUP][
citation needed][/SUP] Modern pregnancy tests still operate on the basis of testing for the presence of the hormone hCG. Due to medical advances, use of a live animal is no longer required.
It is a common misconception that the injected rabbit would die only if the woman was pregnant. This led to the phrase "the rabbit died" being used as a euphemism for a positive pregnancy test. In fact, all rabbits used for the test died, because they had to be surgically opened in order to examine the ovaries.[SUP]
[4][/SUP] While it was possible to do this without killing the rabbit, it was generally deemed not worth the trouble and expense.[SUP][
citation needed][/SUP]
