Home pregnancy tests work by detecting pregnancy hormone in woman s body, a hormone called human Chorionic gonadotropin (HCG). Shortly after a fertilized egg attaches to your uterine lining, woman's body starts to produce this hormone, and home pregnancy tests can predict with quite good certainty whether woman is pregnancy or not. HCG increases in body rapidly after first 2 weeks of pregnancy, so pregnancy test will definitely detect the hormone after 2 weeks of conception. On the other hand, more precise and more sensitive blood test to detect HCG can be done earlier in your doctor's office. Although most home pregnancy tests work in the same fashion, there are different types of tests. For this reason, in case of confusing results, it is recommendable to try another brand, and despite the fact that most tests work in a similar way, always check instructions on the kit, because instructions may vary slightly from kit to kit.