How do pregnancy tests work?

Measure hormones. Pregnancy tests measure the level of a hormone called human Chorionic Gonadotropin (hcg), which begins to be produced when the fertilized egg implants into the uterine wall. This hormone can be measured in urine or in the blood. The blood test is the more accurate of the two methods.
Color. The test strip changes color when there is the presence of certain hormones in the urine released after conception.

Related Questions

How do home pregnancy tests work? Where you urinate on the stick. How is it able to determine if you're pregnant from your urine?

Preg test. It detects the pregnancy hormone hcg. However, it may not detect low levels of this hormone very well, so you should have a confirmatory blood beta HCG level in your doctors office. Read more...