In the first season of Gossip Girl, rumours started flying when Serena van der Woodsen was spotted purchasing multiple home pregnancy tests.

Why so many?

Well, there are at least a few different types of pregnancy tests.

The most common test involves placing urine on a plastic cassette and the result appears in a little window at the end. There are some tests where a stick is dipped into urine. All these tests claim quick results, ease of use and results very soon after a missed period. However, all of these tests in essence utilize the same chemical interactions; so really, Serena could have gotten away with buying only one test for her BFF Blair.

Did You Know?
Up until the 1970s, the standard pregnancy test involved injecting the woman’s urine into a young female rabbit, sacrificing the rabbit a few days later, and then opening it up to examine her ovaries. This is how the popular phrase, “The rabbit died” started, but the same test has also been performed in mice and frogs. You (and PETA) will be glad to know that these days, no animals are harmed in pregnancy testing.

Both home and clinical laboratory pregnancy tests measure the same thing: human chorionic gonadotropin (hCG). When the egg is fertilized, it grows into a blastocyst that implants in the uterus. The blastocyst, and later on the placenta, secretes hCG to maintain high levels of progesterone in the woman’s body.

Did You Know?
Progesterone is one of the hormones used in birth control pills, as it has several effects that make it difficult for fertilization to occur. However, in pregnancy it has many important roles involving the development of the fetus.

The levels of hCG double every 48 hours and shows up in blood and urine. When urine is absorbed on the tip of the plastic cassette-type of test, hCG molecules first encounter a “tracer antibody” that has been made by a mouse cell to recognize and bind one particular part of hCG. This tracer antibody is also labelled with a pink or blue dye and floats around in a liquid part of the test cassette.

Did You Know?
Your body uses antibodies to recognize foreign proteins (for example, proteins from bacteria, viruses and other animals) and alert your immune system of an invasion. There is a different antibody for every protein! Since human proteins are different from mouse proteins, a mouse can make antibodies against human hormones.

The hCG:tracer antibody complex then travels along to the cassette to the results window. This window contains a “capture antibody” in the solid part of the cassette. The capture antibody was made by rabbits or goats and recognizes and binds a different part of hCG. The hCG molecule is consequently “sandwiched” between the tracer and capture antibody. Remember, the tracer antibody is labelled with a dye. A positive results shows up as two blue lines, a check mark, a happy face, etc.

A dipstick test operates very much the same. The stick’s absorbent end contains the labelled tracer antibody. When it is dipped into urine, hCG molecules bind the tracer antibody at the end, and travel up to the other end of the stick containing immobilized capture antibodies. Pregnancy tests with a digital readout have tracer antibodies attached to a chemical that emits an electric signal.

Depending on how much water the woman drank and how many days ago she got pregnant, the test results could vary greatly. Moreover, certain cancerous tumours secrete hCG, which could lead to false positive results. These home pregnancy tests only provide qualitative results — there is either hCG in your urine or not. Another reason why there is no such thing as “a little pregnant.”

Learn more!

hCG reference service

US Department of Health and Human Services

HealthLink BC

Article first published on October 23, 2009.

Photo credit: Wikipedia

About the author:

JoanneHsieh is a Ph.D. candidate in the Department of Biochemistry at theUniversity of Toronto. Her research is on matters of the heart(actually, it’s the link between diabetes and cardiovasculardisease).

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