How do doctors know that hcg isn't detected in pregnancy tests (urine/blood) until implantation?

Before implantation: There is no vascular supply to the fertilized egg. As a result, the HCG cannot be detected in the mother's blood work and urine.

Prior studies: Many studies have been performed to determine when the HCG begins to be produced and when it would reach a detectable level. Those studies tell us that it takes about 7 to 10 days for a urine pregnancy test to become positive. A blood test would be positive days sooner - and at much lower levels.