I started back on my birth control pills this past month, after being off them for 3 months.
My husband and I have not been protecting ourselves from pregnancy. I should have started my cycle 3 days ago, but I only spotted. I worried because I'm having surgery on my knee tomorrow. I know that they will do a pregnancy test, but what if its to soon to show up? Will the surgery in any way harm the fetus?

Stress, blood loss, maybe, and the anesthesia may harm the fetus, talk to your doctors and they'll help you make a decision to wait on the surgery or not, but I think they would be able to determine if you're pregnant or not since your period is officially late.