Updated November 29, 2014.
"The rabbit died," she whispers into the phone
Her husband didn't have to be told, he knew what that meant, there was a bun in the oven!
Around 1927 it was discovered that if you injected the urine of a pregnant woman into a rabbit, there would be corpora hemorrhagica in the ovaries of the rabbit. These bulging masses on the ovaries could not be seen without killing the rabbit to inspect the ovaries, so invariably, every rabbit died, even if the woman wasn't pregnant.
The phrase, "The rabbit died," came to be a euphemism for a positive pregnancy test after the late 1920 and early 1930s.
Today, no bunnies are sacrificed for a woman to find out if she's pregnant. Tests today still look for Human Chorionic Gonadtropin (hCG), we have invented tests that are much easier to perform using blood or urine.
Blood tests are done to determine the presence and quantity of hCG in the blood of a woman. Generally speaking, a level of 5 mIU and above is considered pregnant or positive. This makes a blood test the most sensitive test available at this time. hCG nearly doubles about every 48 hours in early pregnancy. Therefore repeat blood tests can be done to try to determine if the pregnancy is healthy. Using these numbers you can tell not only if the pregnancy is healthy, but sometimes it can detect multiple pregnancies if the numbers are rising at a very fast rate or even some complications like an ectopic (tubal pregnancy) or a molar pregnancy.
The downfall to blood pregnancy tests is that they require medical assistance, cost more than other forms of testing and can cause pain for the mother, particularly if repeat draws of blood are necessary.
There is also more of a wait time involved which can be very stressful for many families.
The home pregnancy test is relatively new considering how long we've been able to test for pregnancy. The test models change dramatically year to year, but the basic premise remains the same - a pregnant woman has hCG detectable in her urine.
The home pregnancy test (HPT) used to be a mini-science lab with several vials containing liquids and powders that you mixed and waited and stirred and waited. Then you poured and waited...
The 4woman.gov FAQ on pregnancy tests explains the older tests, "The first self tests of the 1970's used ring, or "tube agglutination," tests consisting of prepackaged red blood cells to detect HCG in urine. A ring at the bottom of the tube indicated a positive result. Sensitive to movement and human error, ring tests are now rarely used."
Most versions today have one step, mixing urine with monoclonal antibodies which are coated with something to allow them to adhere to hCG if present. The results are indicated by a change of color usually in the form of a line or positive sign. Results are available in as few as two minutes, though the average is around five minutes. The majority of these home urine tests have directions that say you should wait until your are late for your period. This allows the hCG to build up in your body and be excreted in your urine. There is one test, the First Response Early Response test (FRER), that says you may use it three to four days prior to your next period.
The benefits of the home pregnancy test are that they can be done in privacy and at any time you wish to check for pregnancy, within the boundaries of the test instructions. Many of these tests will detect as little as 25 mIU of hCG in urine. Testing for pregnancy can be a stressful time. The newer tests are easier to perform, harder to make mistakes and in general have given us more confidence in our ability to test for pregnancy.
Source:
The Zondek-Ascheim Pregnancy Test. Can Med Assoc J. 1930 February; 22(2): 251–253.