How Do Pregnancy Tests Work?

Anyone who's ever watched television is probably pretty clear on the basics of how to take a pregnancy test: you buy the kit and pee on the stick. But how do pregnancy tests actually work? How does this overpriced piece of plastic actually know whether or not you are incubating a possible future human in your abdomen? Is it psychic? Well, no, not really.

In an animated video from TED-Ed, Tien Nguyen explains everything you've ever wanted to know about pregnancy tests. For example, did you know that the first known pregnancy test existed in Ancient Egypt 1350 BCE and involved urinating on wheat and barley seeds? And that scientists in the 1960s found the test was actually accurate about 70 percent of the time? Of course, nowadays, modern methods are even more effective and accurate, and you can get an answer within a few minutes, rather than having to wait for barely seeds.

So how do the tests work? Well, the key is a hormone called HCG, which is produced early in a pregnancy in order to make sure the body doesn't shed the uterine lining that month. And because it's produced so early, it makes a great marker to use in detecting pregnancies early. Pregnancy tests have three distinct zones: a reaction zone, a test zone, and a control zone.

When the urine hits the test zone, any HCG molecules get bound to the enzymes present on the tab.

Here the enzymes are shown as Y shapes, and the HCG is shown in purple. The enzymes are also attached to dye-activating molecules.

In the test zone, more enzymes are present, and they, too attach to the HCG.

This frees the dye-activating molecules to go and, well, activate the dye.

Which then tells you that, since there are HCG molecules present, you're pregnant.

Finally in the control zone, extra enzymes from the reaction zone should get swept along and activate still more dye. And this should happen whether there's HCG present or not.

That means that if nothing happens, and your pregnancy test doesn't react at all, the test was faulty and you should try another.

For more info on the process, and some insight into why pregnancy tests sometimes aren't fully accurate, you can watch the full video.

Images: Wikipedia Commons; TED-Ed/YouTube (4)