" " While urine is still part of early pregnancy detection, testing animals is no longer included. Yiming Chen/Getty Images

Just as analyzing urine has long played a prominent role in our attempts to understand natural phenomena, so too has the use of animals. From ancient augurs' attempts to divine the future in the entrails of birds to modern AIDS research involving mice and rhesus monkeys, humans have established quite a record of experimenting on other creatures. While often controversial, a number of experiments on animals -- or bioassays -- have led to important breakthroughs in medical science.

In the late 1890s, scientists discovered the existence of hormones. A hormone is an organic product of living cells that regulates specific cellular activities such as growth and reproduction. In the 1920s, scientists pinpointed a specific hormone called human chorionic gonadotropin (hCG). This hormone is found almost exclusively in the blood and urine of pregnant women (though certain cancers also produce it in both sexes). Normally produced by the developing placenta, hCG helps to maintain the pregnancy and support fetal development. The body begins to make the hormone as soon as six days after impregnation, and later stops after the baby is delivered.



Intrigued by these properties, German chemist Selmar Aschheim and gynecologist Bernhard Zondek set out to develop a method of testing for hCG in urine. They decided to exploit the fact that hormones from one animal can generate biological responses in the bodies of other species. Aschheim and Zondek discovered that by injecting female mice with a pregnant woman's urine, they could stimulate the mice ovaries and cause them to go into heat within a few days. They called this test the A-Z Test.

During the 1930s, scientists were able to apply this method to two other species. In South Africa, Lancelot Hogben adapted the A-Z Test for frogs, inventing the Hogben Test. Meanwhile, Dr. Maurice Friedman made his breakthrough with rabbits.

Injecting urine into female rabbits produces much the same results as the A-Z Test. If hCG is present, the urine stimulates changes to the rabbit's ovaries within just a few days. In an act of hormonal trickery, hCG basically fools the rabbit's body into temporarily thinking it's pregnant. As such, the rabbit's ovaries produce temporary tissue structures called corpora lutea and corpora hemorrhagica. Doctors can then spot these growths to confirm the presence of hCG.

While the phrase "the rabbit died" sporadically pops up as an old-fashioned way of saying a woman is pregnant, the urine injection itself didn't actually kill or torment the rabbit. Unfortunately for the rabbit, the fastest way to check the ovaries was to euthanize and dissect the animal. Whether the procedure resulted in a negative or a positive for the patient, it was certainly a test every rabbit failed.

Luckily for the bunnies, the first hemagglutination inhibition test was developed in 1960. This new method was an immunoassay as opposed to a bioassay, meaning it used elements from the immune system instead of living animals. From here, technology gradually improved until the first EPT or early pregnancy test (later known as an error proof test) was approved for home use in 1976. Not only was the EPT more accurate than previous tests, it also provided incredibly fast results. For more information about EPTs, read How Pregnancy Tests Work.

We may have put the rabbit test behind us, but modern EPTs still largely rely on some of the same principles the wheat and barley test employed in ancient Egypt. The tests detect the presence of hCG by pitting human body fluids against other biological elements and observing the results.

For more information about babies, bunnies and modern medical science, explore the links on the next page.

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