Pregnancy tests have come a long way since their inception.

Though take-home pregnancy tests are ubiquitous today, they didn’t exist before the 1970s. Previously, pregnancy tests were conducted in laboratories. Further back still, the pregnancy test du jour in ancient Egypt was for women to urinate in bags of barley and wheat seeds. If either of the seeds sprouted quickly, this indicated pregnancy. This method had a surprising success rate of 70-85% – scientists believe this is because the estrogen in the urine stimulated the seed growth. Thankfully, women today don’t have to urinate in bags of seeds to determine whether they’re pregnant. Today’s home pregnancy tests are so accurate that doctors often won’t need to follow up a positive result with further confirmatory tests.

With that brief historical interlude out of the way, let’s dive into how pregnancy tests work.

Pregnancy tests search for the presence of human chorionic gonadotropin (hCG). This is produced by the cells in the placenta when the developing embryo attaches to the uterus, between one and two weeks after conception.

The primary function of hCG is to support the corpus luteum, which serves the vital role of producing estrogen and progesterone during pregnancy.

hCG levels increase as pregnancy progresses, peaking between weeks eight and eleven. In the early stages, therefore, its presence may not be detected by pregnancy tests. After implantation, though, hCG levels double approximately every 48 hours, easing detection and improving the accuracy of tests.

hCG appears in the mother’s urine. Hence, this is what is used in the testing process. As the woman urinates on an indicated pad on the test stick (the reaction zone), the hCG present in the urine binds to antibodies. Contained within these antibodies are enzymes that form a series of reactions along the test strip. These enzymes in turn cause the dye molecules on the test strip to change color. This line only appears if the urine contains hCG – and hence, if the woman is pregnant.

Another line appears on the strip regardless of whether the woman is pregnant or not. This confirms that the test is functional.

In recent years, digital pregnancy tests have become increasingly common. Though they look more sophisticated, their inner mechanics are largely the same. The only difference is that a sensor is used to read the results from the strip and display ‘pregnant’ or ‘not pregnant on screen.

Pregnancy tests can be something of a black box. Hopefully, this post allays some of your anxiety about what exactly is going on when you use them.