How fast can you tell if your pregnant ?

Quickly: With home pregnancy testing it's possible to determine on the first day of your missed period. It's possible to test sooner but that would require blood testing which usually isn't done.

Within 10 days: A pregnancy test measures a hormone in the body called human chorionic gonadotropin (HCG). HCG is a hormone produced during pregnancy. It appears in the blood and urine of pregnant women as early as 10 days after conception. For more information, refer to: http://www.nlm.nih.gov/medlineplus/ency/article/003432.htm

Pregnancy test: It takes a week for pregnancy tests to turn positive after a woman conceived.