The Secret
Behind That Stick

A pregnancy test can change your life in a matter of minutes. But how does a pregnancy test actually work?

Woman with early pregnancy test

If you've ever browsed for a pregnancy test, you know there are several types, and many claim the ability to tell whether or not you're pregnant even before you miss a period. There's no question that a pregnancy test holds the valuable answer to a pressing question, but have you ever wondered how pregnancy tests work?

While the outcome of that little stick's answer can be life-altering, the science behind a pregnancy test is actually quite basic, regardless of the type of test you choose. Once you conceive, your body starts to produce a hormone called human chorionic gonadotropin (hCG) -- usually about six days after the egg and sperm join forces. Once its produced, your hCG levels increase rapidly each day thereafter. Plain and simple, pregnancy tests are based on detecting this hormone. The catch, however, is that not every woman or pregnancy produces hCG at the same rate. According to the U.S. Department of Health and Human Services Office on Women's Health, "studies show that in up to 10 percent of women, implantation does not occur until much later" -- which may not be until after first day of the missed period.

Some pregnancy tests differ in their sensitivity to hCG levels, and in how much of the hormone the test needs to find to give an accurate reading. If you're testing before you've had any symptoms because you used a fertility monitor and ovulation test to increase your chances of conception, choose a test that is specifically designed to read very slight hCG levels (usually indicated by the amount of days before or after a missed period when the test can detect the hCG).

Regardless of their sensitivity, most at-home pregnancy tests "read your urine," which you'll collect either by holding a stick into your "stream," or collecting it in a cup and dipping the pregnancy test stick into it. Read the directions carefully on how to effectively execute every pregnancy test you buy, as each brand varies in its method, indicator window and a potential "control window."

Once you've tested, lay the stick flat and wait for the result -- which can take one to several minutes, based on the test. If you've followed the directions and get a negative result, but still have no period after a few days, test again. Depending on the test and your body, some negative readings are generated because the test simply could not detect the hCG levels yet. This is the reason that it is not uncommon to get a "false negative" reading when you are, in fact, pregnant.


According to the American Pregnancy Association, about 85 percent of normal pregnancies will have hCG levels that double every 48 to 72 hours. If you suspect that negative pregnancy test is really a positive, wait that amount of time to let hormone levels increase before you test again.

More on pregnancy

Tips for tracking your fertility
5 Surprising signs of pregnancy
Ways to improve your chances of conception


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