Home pregnancy tests let you know if you are pregnant, or not. These tests detect the presence of the pregnancy hormone called human chorionic gonadotropin (hCG) in your urine. A blood pregnancy test can be done in your doctor's office and it is usually positive 3-4 days before the home urine pregnancy test.
hCG is produced by cells that will eventually develop into the placenta. It first enters your bloodstream when the fertilized egg starts to implant in the lining of your uterus, on average 9 days after ovulation/fertilization and between a 6-12 days range. The amount of hCG in your body then increases rapidly over the next few weeks, often doubling every 2-3 days or so. When a test detects the hormone in your urine, it will show you a positive result.