What is Biology Good For?
A 'Yes or No' in Just 3 Minutes: Home Pregnancy Tests

(This assignment is optional. Read this essay and answer the questions at the bottom for 3 extra credit points. You may only turn in one per six weeks. The assignment is due one week before the end of the six weeks. It is not necessary to visit the links in the text unless you are interested in more information.)

In 1977, the Food & Drug Administration (FDA) approved the first Home Pregnancy Test. The Home Pregnancy Test checks a woman's urine for human chorionic gonadotropin, (hCG), a hormone made by the developing baby after pregnancy occurs. The concentration of hCG in the urine doubles every 2 - 3 days, reaching a maximum at about the 8th week of pregnancy. (This same hormone gives the newly pregnant mother the lovely feeling of morning sickness...)

How does the Home Pregnancy Test work? The test depends on the use of antibodies - Y-shaped proteins that an immune system makes normally against invaders - bacteria, viruses, or foreign tissue. Every antibody is highly specific to only a certain specific molecule, called an antigen. Thus, for every antigen, a perfectly fitting antibody can be made by the body to recognize only that antigen. Humans as well as all other mammals make antibodies to foreign substances. To make the Home Pregnancy Test, mice are injected with purified hCH (a foreign protein - an antigen - to mice), and a few weeks later the antibodies that they made to hCH arere isolated. [Image]

Home Pregnancy Tests use 3 different types of antibodies: Two from mice and one from goats. The first antibody molecules are placed in the papery material in the test strip (where the urine sample is applied). When a urine sample is applied, hCG in the urine binds to the mouse antibodies and starts to travel up the paper into the test region. Antibodies that do not bind to hCG (there are always more antibodies than there is hCG) also move into and through the test region, where they serve as a positive control.

In the Test Region, a second antibody to hCH (fixed to the paper so that it cannot move), stops the hCG and the original antibody bound to it. This is in the first window of the pregnancy test. Any antibodies that are not bound to hCH ('unbound' antibodies) continue to move up the test strip into the Control Region.

In the Control Region, (the second window of the pregnancy test) the third antibody is encountered. This antibody was isolated from goats that were given an injection of mouse antibodies. The goat immune system recognizes the mouse antiboby as a foreign protein, and makes an antibody to the mouse antibody!! The 'unbound' mouse antibodies are recognized by the goat antibodies fixed in the paper, and they stop moving at the second window.

At this point (the test has been at work for roughly 3 minutes), an enzyme attached to the first mouse antibody converts a colorless chemical placed in the test window and in the control window to a deep pink color. If the woman has hCG in her urine, the hCG-mouse antibody will be found in two places: the test region and the control region. There will be two color reactions, one in each window. If this occurs, the woman is pregnant, and should make an appoinment to see her Doctor to begin early prenatal care. (If a woman was taking fertility drugs, this might give a false positive test, but otherwise, the test is very reliable).

If the woman is not pregnant, the antibodies will not stop at the first window and instead will all move into the second window. In this case, the woman sees only one color reaction, in the control window. This lets her know that antibodies in the Home Pregnancy Test were working correctly, even if she was not pregnant. A negative test might also mean that the woman did a pregnancy test too soon after conception, and should perhaps repeat the test in 1 week to see if she obtains the same result. This is why many home pregnancy tests come in packages of two!

There are over 100 Home Pregnancy Tests on the market today. Home Pregnancy Tests differ in the amount of hCG that they can detect. The most sensitive (and expensive) tests can detect a positive pregnancy just 10 days after conception. This is before a woman even misses her period! Sometimes different tests have different color reactions in blue, or in the shape of a "+" to indicate a positive test. All, however, rely on the use of antibodies to detect hCG.

Sales of Home Pregnancy Tests exceed $250 million a year. This corresponds to about 20 million pregnancy tests per year, making the Home Pregnancy Test the most popular home diagnostic test available.

Other Home Diagnostic Tests include glucose detectors (1980) for diabetics, ovulation predictors (1984), home cholesterol tests, tests for the presence of illegal drugs (1998), and tests to detect Hepatitis B and C, and the HIV / HTLV viruses, involved in the disease AIDS .

Material and images for this Good For came from the article "Home Pregnancy Tests" by Rebecca Lipsitz, in the November 2000 issue of Scientific American


Extra Credit Questions: Please answer the following questions on a piece of paper and turn it into me.

1. What would be one benefit to the developing child from having an early pregnancy test?
2. Why are there two windows in a pregnancy test rather than just one?
3. Why does only one window turn color if a woman is NOT pregnant?