1. How many days does it take for test strips to detect pregnancy? Pregnancy test strips determine whether you are pregnant by detecting the level of human chorionic villi (hCG) in urine. hCG is a hormone secreted by embryonic tissue after the fertilized egg implants. Typically, pregnancy test strips can be used to test one week after your menstrual cycle is expected to be delayed. However, some test strips can test a few days before your menstrual cycle is expected to start.