In the TED-Ed video lesson, “How do pregnancy tests work?” Tien Nguyen explains how each modern pregnancy test is able to tell if you’re pregnant within minutes. The concept of the modern pregnancy test started in 1963 when a small study reproduced the ancient Egyptian pregnancy test, where women would urinate on wheat and barley seeds and wait to see if they sprouted. If wheat sprouted faster, they believed it was a girl, but if barley, a boy. The 1963 study was able to predict pregnancy with a 70 percent accuracy rate, although it can’t reliably tell the sex of the baby.
Although many home pregnancy tests claim to be accurate as early as the first day of a missed period or even before, says the Mayo Clinic, it’s best to wait until one week after a missed period. OTC pregnancy tests are all designed to do one thing, detect a hormone called human chorionic gonadotropin (HCG), which is produced in the earliest stages of pregnancy and tells the body not to shed the inner lining of the uterus that month. The hormone enters the bloodstream and urine when it begins production in the placenta, and increases rapidly every two to three days.
The over-the-counter (OTC) pregnancy test begins when urine is applied to the exposed end of the strip as the fluid travels up the absorbent fibers that will cross three separate zones. The first zone, the reaction zone, is where the y-shaped proteins called antibodies — they contain a handy enzyme with the ability to turn on dye molecules — will latch onto any HCG.
The urine then picks up all the AB1 enzymes and carries them to the test zone, where the results show up. This type of test is known as the sandwich assay. If HCG is present, it gets sandwiched between the AB1 enzyme and AB2 and sticks to the test zone, allowing the attached dye-activating enzyme to create a visible pattern. If there’s no HCG, the wave of urine and enzymes just passes on by.
See a video that Nguyen presented with his talk: