Here's why the timing matters: When a person becomes pregnant, a hormone called human chorionic gonadotropin (aka hCG) is released into their bloodstream, and then makes its way into their urine, according to Medline Plus. Home pregnancy tests are designed to pick up on the presence of hCG, while pregnancy tests that you have done in the doctor's office can determine the amount of hCG in your blood (which indicates how far along the pregnancy is). At first, the amount of hCG in a pregnant person's urine increases at a rapid rate of about 50% each day . So, if you take the urine test before your body has produced enough hCG for it to be detected in your pee, then you could end up with a false negative, according to the Mayo Clinic