If embryo isn't connected to maternal blood supply until 7days post conception, this would mean hcg isn't yet produced for a urine pregnancy test?

No: The embryo can produce HCG as early as 5 days post fertilization but the amounts are so small and the maternal-fetal blood connections are so minimal that not even a blood test can pick it up, let along a urine pg test that is not positive until your HCG levels are 20 iu/ml or more. You should, however, be able to pick up HCG in a blood test as early as day 9 post ovulation but the levels are low.