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.