How Do Home Pregnancy Tests Work?

hands holding pregnancy test kit

Home pregnancy tests provide a fast, convenient way to find out if you’re expecting. But how do over-the-counter pregnancy tests actually work to detect the hormone hCG? Here’s a look at what goes on inside those plastic sticks.

Pregnancy Tests Detect hCG

Human chorionic gonadotropin (hCG) is the key pregnancy hormone tested for in urine and blood. Produced after an embryo implants, hCG levels double every 48 hours in early pregnancy. This exponentially increasing hCG is what home tests analyze.

Antibodies Detect the Hormone

The absorbent tip of pregnancy tests contains antibodies that specifically react to and bind with hCG molecules. These antibodies are chemically trained to detect only the unique structure of hCG.

Capillary Action Draws Urine Up

When urine is absorbed into the pregnancy test stick, capillary action draws the sample up over the reactive antibodies. Urine acts like a transportation medium bringing any hCG molecules from your body to the antibodies.

Antibodies Bind to hCG

If hCG is present in the urine, the antibodies will bind to and capture the hCG hormone. The more hCG present, the more binding complexes that will form.

Colored Line Appears

The binding of an antibody to hCG triggers the release of a colored dye. This dye flows across the Result window to create a visible colored line. No hCG results in no binding and no line color.

Test Line vs Control Line

Home pregnancy tests have two result lines. The Control line ensures the test is working properly. The Test line will only appear if hCG is detected in your urine by the antibodies.

Evaporation Can Cause Evap Lines

As urine evaporates within the test window, evaporated dye can sometimes create a faint line known as an evaporation or evap line. Only colored lines that form within the time limit are true positives.

Sensitivities Vary

Tests measure hCG in international units (IU). Sensitivity thresholds for positive results range from 20-100 IU depending on test brand. Early tests can often detect lower hCG.

Easy But Complex Technology

While home pregnancy tests seem simple, the antibodies and detection chemistry allow these tests to capture tiny hormone traces and create dramatic colored lines symbolizing a new pregnancy chapter ahead!

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