How soon after can you find out if you are pregnant once you had sex?

7 days late period: To be certain, you should wait until your next period is 7 days late, buy a fresh pregnancy test and use it as directed. Some early pregnancy tests will advertize that they can reliably detect pregnancy on the first day of a missed period, but they don't always perform that well. If unprotected sex was today or yesterday you could look into the morning-after pill. Ask your dr. Or pharmacy.

Pregnant? : First, you must have "missed" a period by at least a week, do depending on what your menstrual cycle is, use a "home pregnancy test" about a week after your period is expected. If negative, get a serum HCG test after another week has passed.